BAGHDAD -- Signs are growing that stretches of Iraq and Syria are morphing into a single battlefield for militants, exacerbating Iraq's slide into renewed deadly chaos a year and a half after U.S. troops pulled out.
Iraqi border posts along the Syrian frontier are coming under attack, and Syrian truck drivers have been singled out and shot inside Iraq. Syrian soldiers earlier this year sought refuge across the border, only to be massacred by al-Qaida.
Combat-hardened Iraqi fighters, meanwhile, are crisscrossing the frontier. Al-Qaida-linked Sunni militants are cooperating with hard-line Islamists among the Syrian rebels, while Iraqi Shiite fighters are joining militiamen from Lebanon's Hezbollah to fight alongside forces loyal to Syrian President Bashar Assad's Iranian-backed regime. U.S. officials believe Iranian arms are still being shuttled to Damascus through Iraqi airspace.
"What is going on in Syria has a big, clear impact on us ... especially since there are attempts to move the battle to Iraq," said Ali al-Moussawi, spokesman for Iraqi Prime Minister Nouri al-Maliki.
It's happening as the deadly drumbeat of violence is growing louder across Iraq. Nearly 2,000 lives have been violently snuffed out across the country since the start of April.
The extent of the killing hasn't been this bad for half a decade, when Iraq's last tip toward civil war was easing and American troops were still here to help keep the peace. Attackers killed more than 60 people in a relentless wave of bombings on Monday. Another nine were slain Tuesday.
"The events in the past three or four months prove that the violence in Iraq and Syria are two sides of the same coin," said Haider Ayed, a 35-year-old math teacher and father of two in Baghdad's southwestern Bayaa neighborhood. "We are going through a very dangerous period."
It's a worrying trend for the United States, which is mulling whether to arm Syria's rebels even as it adapts to a new relationship with Iraq following a divisive war that claimed nearly 4,500 American and more than 100,000 Iraqi lives.
The spokesman for the American Embassy in Baghdad, Frank Finver, said the U.S. shares Iraqi government concerns about the level of violence in Syria, as well as about extremists who are trying to capitalize on the situation in Syria and incite violence inside Iraq.
The U.S., Finver added, is working with allies and moderate members of the Syrian opposition to isolate extremists and "ensure their violent and divisive ideology does not take root in Syria or spill over into Iraq."
Iraq officially remains neutral in the Syrian conflict. Al-Maliki has repeatedly called for a peaceful, political solution to the crisis, though he has also warned that a victory for the rebels would unleash sectarian war in Iraq and Lebanon.
On Tuesday, Iraqi Foreign Minister Hoshyar Zebari insisted that Iraq has no official or military role in the civil war, and said Baghdad does not encourage the movement of any Iraqi fighters to Syria.
Still, the cross-border violence continues. An Iraqi border guard was killed and two others were wounded Sunday in clashes with fighters the Interior Ministry said were members of the Free Syrian Army rebel group. Border guards thwarted two other attempts by gunmen and smugglers to sneak into Iraq from Syria, officials said.
The long and porous border runs along Iraq's Sunni-dominated provinces of Anbar and Ninevah, and was a key conduit for arms and al-Qaida fighters in the years following the 2003 U.S.-led invasion. Centuries-old cultural and tribal affiliations span the loosely defended desert frontier.
A Western diplomat in Baghdad described Iraq's control of the border with Syria as limited.
"With all the air power and surveillance the (U.S.-led) coalition had, it did not fully control that border," said the diplomat, who spoke on condition of anonymity because he is not authorized to speak publicly. "On the border area there is a nexus of smuggling and informal links and trade."
Sadoun al-Shaalan, a provincial councilman in Anbar province, said clashes along the border between Iraqi forces and gunmen from Syria are growing more frequent. He attributed the uptick to a rise in smugglers profiting from the war as well as insurgents shuttling fighters back and forth.
Iraqi army units deployed near urban centers within the province - where anti-government sentiment is strong -- are often unwilling to confront insurgents deep in the desert because they lack sufficient aerial support and experience in the harsh, remote environment, he added.
"Most of the time, the gunmen and smugglers have better weapons and equipment than our units," he said.
Iraq's isolated western desert was the scene of the country's deadliest incident of spillover from the Syrian conflict -- a March attack in which 51 Syrian soldiers were killed. The Syrian troops had retreated into Iraq after their border post was attacked by rebels, and were later ambushed, along with their Iraqi military escorts, in a highly organized assault involving explosives, gunfire and rocket-propelled grenades unleashed by al-Qaida's Iraq arm.
The militant group, known as the Islamic State of Iraq, linked the mission directly to the conflict in Syria, saying it planned the raid following "the blessed operations carried out by our brothers in Syria."
The group has since attempted to frame its cause as part of a broader cross-border battle. Leader Abu Bakr al-Baghdadi went so far as to announce a merger in April with Syria's Jabhat al-Nusra, the most powerful rebel force fighting to topple Assad. Al-Nusra leader Abu Mohammad al-Golani quickly distanced himself from that takeover attempt.
Al-Qaida's central leader, Ayman al-Zawahri, has tried to end the squabbling. He released a statement this week ordering the two groups to remain separate and not to attack one another, while saying both of their leaders could keep their posts.
Syrians were also targeted in Iraq earlier this month when gunmen set up a fake checkpoint on a main highway linking Baghdad to Syria and Jordan. The gunmen killed three Syrian truck drivers and burned their rigs.
Al-Shaalan, the provincial council member, said the attackers managed to operate their checkpoint for a full hour and then got away with several hostages without any resistance from the Iraqi military.
Anthony Cordesman, a longtime observer of Iraq as an analyst at the Center for Strategic and International Studies, questions the lack of American attention on Iraq, particularly in light of Washington's efforts to isolate Iran, Syria's main patron.
"For all the current attention to Syria, Iraq is the larger and more important state," he wrote in a commentary this month. A slide toward civil war inside Iraq will push its majority Shiites closer to Iran and Syria, he predicted.
"If Assad survives and the Arab Gulf states continue to isolate Iraq, the largely token U.S. presence in Iraq is likely to become irrelevant and Iraq is likely to become part of a Shiite axis going from Lebanon to Iran," Cordesman wrote. "If Assad falls ... Iran seems likely to do everything it can to replace its ties to Syria with influence in Iraq."
-- Associated Press writers Qassim Abdul-Zahra, Sinan Salaheddin and Sameer N. Yacoub in Baghdad contributed to this report.