Two U.S. service members were among four Americans killed in the contested northern Syrian town of Manbij on Wednesday in a bombing claimed by the Islamic State, a U.S. military spokesman said.
In addition to the two troops, whose military service was not identified, one Defense Department civilian and one American contractor supporting defense operations were killed, officials with U.S. Central Command announced. Three more troops were wounded.
The explosion happened while the troops and American workers were "conducting a local engagement" in Manbij, officials said.
"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," a statement noted.
The names of those killed in the blast will not be released until 24 hours after next-of-kin are notified, per military policy.
Reuters, in a report from Beirut citing a U.S. official, had earlier said that four U.S. troops were killed and three others were wounded when a suicide bomber set off an explosive vest near a restaurant in a Manbij town market.
The Associated Press, which first reported on the explosion, said ISIS claimed that a suicide bomber set off the blast with an explosive vest.
According to the Syrian Observatory for Human Rights, the London-based group monitoring Syria's civil war, and the local Manbij Military Council, more than a dozen people were killed or wounded in the attack.
Before the explosion Wednesday, a total of four American troops had died in Syria since the U.S. began training, advising and equipping the Syrian Democratic Forces (SDF) in 2015 -- two in combat and two from non-combat causes, according to the Pentagon.
The U.S. has been conducting joint patrols with the Turkish military since early December under an agreement worked out by then-Defense Secretary Jim Mattis to ease Turkish concerns about the U.S.-backed SDF's presence in the region.
According to the Syrian Observatory, several SDF members were also killed in the explosion Wednesday.
Videos from the scene released by local news agencies showed a shattered restaurant and the street outside littered with blood and debris.
White House Press Secretary Sarah Sanders tweeted that President Donald Trump and Vice President Mike Pence had been briefed on the Manbij attack. There was no immediate word on whether or how the attack might affect the planned withdrawal of the estimated 2,000 U.S. troops in Syria.
On Dec. 19, Trump made the surprise announcement that U.S. troops would be withdrawn.
In a White House video at the time, he said, "We have won against ISIS. Now it's time for our troops to come back home."
The following day, Mattis issued a letter of resignation, saying Trump as commander-in-chief deserved a defense secretary whose views are "better aligned" with his own.
Last Friday, CJTF-OIR and the Pentagon confirmed that the withdrawal from Syria had begun but was being temporarily limited to the pullout of equipment. In a statement, a Pentagon spokesman stressed that no personnel had as yet been withdrawn and gave no timetable for the completion of the withdrawal.
Operation Roundup, the U.S.-backed offensive by the SDF against the last remnants of ISIS in Syria that began last May, is continuing against ISIS fighters in eastern Syria near the Iraqi border with U.S. air and artillery support, according to the task force.
The SDF ousted ISIS from Manbij, about 10 miles from the Turkish border, in August 2016 following a two-month siege. The U.S. hailed that victory as a significant milestone in the ultimate defeat of ISIS in Iraq and Syria.
The Manbij operation also set the stage for the SDF's march south to take Raqqa, the so-called capital of the "caliphate," in October 2017 following a lengthy siege.
From the outset, Turkey, a NATO ally, has protested the presence of the SDF in Manbij and throughout northeastern Syria, adding to the complicated and risky political and military consequences of the planned U.S. withdrawal.
In recent months, Turkish President Recep Tayyip Erdogan has renewed threats to have the Turkish military and its allied local militia, the Free Syrian Army, attack Manbij and clear the region of the SDF.
Turkey has also demanded that the U.S. hand over its bases and observation posts in the region. Last week, White House National Security Adviser John Bolton went to Ankara seeking agreements on conditions for the U.S. withdrawal and guarantees not to attack the SDF, but Erdogan refused to meet with him.
In a series of tweets Sunday, Trump suggested the creation of a buffer zone in northern Syria to protect the mixed Syrian Kurdish and Arab SDF from threatened attack by Turkey.
"Create 20 mile safe zone" in northern Syria, he tweeted. "Likewise, do not want the Kurds to provoke Turkey."
Trump also said that attacking the SDF would be against Turkey's long-term economic interests. "Will devastate Turkey economically if they hit Kurds," he said.
Trump gave no timetable, but said that, once the troop withdrawal is complete, the U.S. could attack from a base outside Syria if ISIS attempted to regroup.
"Starting the long overdue pullout from Syria while hitting the little remaining ISIS territorial caliphate hard, and from many directions," he said on Twitter. "Will attack again from existing nearby base if it reforms."
Turkey views the SDF as dominated by the Kurdish YPG (People's Protection Units), the main fighting force within the SDF.
Erdogan and other Turkish officials have repeatedly charged that the YPG is allied with the PKK (Kurdistan Workers Party), the separatist group in eastern Turkey that has been labeled a terrorist organization by the U.S., Turkey and the European Union.
In a session with Pentagon reporters in late November, Mattis rejected the Turkish charge that the YPG is allied with the PKK.
Add after last graft existing story ending -- with the PKK.
It was not immediately clear whether U.S. troops were directly targeted, but the Syrian Observatory said in a later report that the attack occurred at the al-Omaraa restaurant, which is known as an occasional stop-off point for U.S. troops on patrol in Manbij.
The Syrian Observatory said the death toll in the bombing had risen to at least 16, including five members of the SDF, and several of the wounded were in critical condition.
At a Pentagon welcoming ceremony for Japan's defense minister, Acting Defense Secretary Patrick Shanahan extended condolences and "our thoughts and prayers to the families and team members of those killed and wounded during today's attack in Manbij."
"Our fight against terrorism is ongoing, and we will remain vigilant and committed to its destruction," he said, adding that the attack is "a stark reminder of the dangerous missions that men and women in uniform perform on our behalf each and every day."
Shanahan did not respond to a question on whether the attack would affect the pace of the planned U.S. withdrawal, according to a Pentagon pool report.
In Moscow, Russian Foreign Minister Sergey Lavrov said at his annual January news conference that his country is prepared to back a buffer zone in Manbij and other areas of northeastern Syria, but only if the area comes under the control of Syrian President Bashar al-Assad's forces.
Parts of Syria that are "currently under U.S. control" must come under the control of the Assad regime "after the withdrawal of U.S. troops," Lavrov said, according to Russian media reports.
He said Russia would be against Turkey setting up a "security zone" in the region.
"We are convinced that the best and only solution is the transfer of these territories under the control of the Syrian government, and of Syrian security forces and administrative structures," Lavrov said.
Erdogan said Wednesday that the Manbij attack could have been intended to delay the U.S. withdrawal from Syria, Turkey's state-run Anadolu news agency reported.
At a news conference with visiting Croatian President Kolinda Grabar-Kitarovic, Erdogan also said that Turkey is prepared to take over the fight against ISIS from the U.S.
"Turkey will continue the fight against Daesh [ISIS] and eliminate this terrorist group in Syria," he said.
-- Richard Sisk can be reached at Richard.Sisk@Military.com.