BAGHDAD -- The wounded Syrian government troops were returning to their country in trucks escorted by Iraqi soldiers. They'd almost reached the border, near the frontier town of Akashat, when the attackers struck.
Regional intelligence officials saw the March 4 ambush, which left 48 dead, as evidence of a growing, cross-border alliance between two powerful Islamic extremist groups -- al-Qaida in Iraq and Jabhat al-Nusra or Nusra Front in Syria. Nusra Front is the most effective rebel faction fighting President Bashar Assad's regime, and the U.S. designates both Sunni jihadi groups as terrorist organizations.
Iraqi intelligence officials say the burgeoning cooperation is pumping new life into the Sunni insurgency in their country. They point to nearly 20 car bombings and suicide attacks that killed over 65 people, mostly in Baghdad, on the eve of the 10th anniversary of the U.S.-led invasion of Iraq last month.
The alliance is also nurturing Nusra Front, which emerged as an offshoot of Iraq's al-Qaida branch in mid-2012 to battle Assad's regime as one of a patchwork of disparate rebel groups in Syria. Nusra Front's presence on the battlefield complicates desperately needed international support for Syrian rebels because foreign backers do not want to bolster Islamic extremist groups.
Two Iraqi intelligence officials said the cooperation reflected in the attack on the wounded Syrian troops prompted their government to request U.S. drone strikes against the fighters. They spoke on condition of anonymity because they were not permitted to talk to reporters about the subject.
A U.S. official confirmed that elements within the Iraqi government had inquired about drone strikes. But the official said the U.S. was waiting to respond until the top level of Iraqi leadership makes a formal request, which has not happened yet.
Iraq is also turning elsewhere for assistance. Ministry of Defense spokesman Staff Lt. Gen. Mohammed al-Askari said that in Iraq's last weapons deal with Russia, Baghdad requested aircraft and heavy weapons to try to seize control of the Iraqi-Syria border region where the groups are operating.
The two Iraqi intelligence officials said the jihadi groups are sharing three military training compounds, logistics, intelligence and weapons as they grow in strength around the Syria-Iraq border, particularly in a sprawling region called al-Jazeera, which they are trying to turn into a border sanctuary they can both exploit. It could serve as a base of operations to strike either side of the border.
"We are very concerned about the security situation in Iraq," said Iraqi government spokesman Ali al-Moussawi. He said Iraqi ground troops and the country's tiny air force were unable to quell the militant activity in the border zone.
"This area is a nest of terrorist cells," he said.
A Jordanian counterterrorism official said al-Qaida in Iraq was assisting Nusra Front "with all possible means, including weapons, fighters and training."
Another regional security analyst cited the attack on the wounded Syrian troops in Iraq as decisive proof of cooperation.
"This is operational collaboration," the analyst said, requesting anonymity because of the sensitivity of the subject. "The transfer of weapons, tactics and ideas, what they call complex suicide attacks."
Iraq and Syria's other neighbors, Jordan, Lebanon, Turkey and Israel, all fear the spillover effects of the 2-year-old civil war. Iraq, Lebanon and Syria all share a similar, fragile ethnic mix and the concern is that the conflict could cause sectarian warfare between Sunnis and Shiites to spread throughout the region.
Under Iraqi Prime Minister Nouri al-Maliki's Shiite-led government, already tense relations with minority Sunnis have worsened. There are also longstanding strains between Arabs and Kurds, who control their own autonomous region in Iraq's north.
In Syria, Assad is a member of the Alawite sect, an offshoot of Shiite Islam, and his security forces are heavily stocked by fellow Alawites and Shiites. But Alawites are a minority, and the opposition fighting him is predominantly made up of majority Sunnis.
Shiite-dominated Iran and Shiite militant group Hezbollah in Lebanon are Syria's two closest allies in the Mideast.
Iraq pledged on Friday that it would conduct more searches of Syria-bound planes and vehicles, days after visiting U.S. Secretary of State John Kerry asked al-Maliki to stop shipments of Iranian weapons and fighters through Iraqi territory to help Assad's regime.
But al-Moussawi also pointedly noted that Iraq would try to halt weapons shipments to rebels.
Nusra Front's role in Syria's civil war is troubling not only for Iraq but for international supporters of the Syrian opposition as well.
Since it emerged in mid-2012, it has transformed into the most potent fighting force among rebel groups, with a strong presence in the eastern provinces of Raqqa, Deir el-Zour and Hassakeh close to the Iraqi border.
The group has claimed responsibility for many of the deadliest suicide bombings against the regime and military facilities. Its success has led to popularity among fighting groups, though a source of friction with more moderate and secular brigades in Syria. Nusra Front has complicated the fractured Syrian opposition's cause and remains a chief reason the U.S. has been reluctant to arm the Syrian rebels.
Intelligence officials estimated last month that about 750 Nusra Front militants - including foreign fighters from other Arab countries - were among approximately 2,000 anti-Assad fighters who control long stretches of borderlands on the Syrian side. The officials said the Syrian militants were increasingly crossing into Iraq to meet their al-Qaida counterparts.
They mostly operate from the al-Jazeera region that straddles three provinces of western Iraq. The region abuts part of the porous, 375-mile border, composed of desert valleys, orchards and oases.
Their cooperation with al-Qaida intensified when Nusra Front seized control of two border crossings between Syria and Iraq, freeing up space for the militants to operate, the Iraqi intelligence officials said.
The rebels seized the Rabia-Yarubiya crossing in March and the al-Qaim crossing in September, according to a report on Nusra Front by the U.K.-based Quilliam Foundation. One crossing still remains in Syrian hands -- the Walid-Tanf post near where the Syrian, Jordanian and Iraqi borders intersect.
Government spokesman al-Moussawi and Jassim al-Halbousi, a provincial council member in Iraq's Anbar province, also confirmed the two groups were using "nests" -- Arabic slang for small bases -- in the area.
"This battle has two directions, from Syria to Iraq and from Iraq to Syria," said analyst Mustafa Alani of the Geneva-based Gulf Research Center.
The Jordanian counter-terrorism official said al-Qaida in Iraq was also providing "expertise and logistics" to the Nusra Front.
"During training, Nusra elements are taught how to fire rockets and machine guns, maneuver in the desert terrain and handle arms supply to its in-the-field fighters," he said, requesting anonymity because he was not authorized to speak to the media.
He said the training is conducted in "temporary camps in a no-man's land along the Syrian-Iraqi border." After training sessions, the camp is usually dismantled so as not to leave traces behind.
"It's natural for al-Qaida to help another group with a similar ideology," he said. "The aim is to control the street in Syria as a step toward toppling Assad and setting up an Islamic jihadi state there."
According to the Iraqi officials, the group is helping al-Qaida expand in western Iraq and conduct high-profile attacks against mostly Shiite targets.
A wave of daring and coordinated strikes in March led intelligence officials to conclude that al-Qaida militants had strengthened their weapons-smuggling networks as well as their ability to find volunteers and carry out attacks.
They said the surge was caused by increased cooperation with Nusra Front fighters who appear to have facilitated the flow of suicide bombers, weapons and explosives into Iraq.
The ambush on the wounded Syrian troops only strengthened the notion of cooperation. An intelligence official said attackers appeared to have been tipped off.
The soldiers were making their way back to Syria in an Iraqi-escorted convey traveling hundreds of miles westward.
The assault began with militants detonating explosive charges on the military escort vehicles assigned to protect trucks carrying the Syrian soldiers, al-Qaida in Iraq claimed in a statement posted on its website after the attack.
After that, "the fighters launched an attack from two directions using light- and medium-range weapons as well as rocket-propelled grenades," it said. "Within less than half an hour, the whole convoy ... was annihilated."
U.S. and Iraqi forces had mostly quelled al-Qaida's presence in Iraq before American troops withdrew in late 2011. But by September 2012, Iraqi intelligence officials were warning that al-Qaida was regrouping, seizing on regional instability and government security failures to regain strength.
They reported at the time that fighters linked to al-Qaida were crossing into Syria to battle the Assad regime. Since then, those fighters have strengthened the al-Jazeera area into what they hope will be a haven to battle their foes, intelligence officials said.
"For these guys," said the regional security analyst, "the border between Iraq and Syria is not even a real thing."
-- With reporting by Jamal Halaby in Amman, Zeina Karam in Jordan and Lara Jakes in Washington.