More than 40 Syrian soldiers fleeing rebel fighters were killed in an ambush in Iraq as the Iraqi military drove them back to Syria in buses, Baghdad said.
"Armed groups from the Iraqi and Syrian side" coordinated the attack, which also killed at least seven Iraqi military personnel, Iraqi Prime Minister Nouri al-Maliki said through a spokesman, who said Iraq would now deploy more security forces on the border.
It was the first such killing of Syrians in neighboring Iraq since the Syrian uprising started two years ago this month.
Spokesman Ali al-Musawi did not say which armed groups he considered responsible for the attack by gunmen armed with mortars, automatic weapons and improvised bombs. However, The New York Times said it was clear he meant Sunni militant extremists affiliated with al-Qaida in Iraq.
Maliki is a Shiite accused by critics of trying to marginalize Iraq's Sunni population after the U.S. occupation of Iraq ended in 2011.
Maliki has not said he supports Syrian President Bashar Assad. But he said last week a Syrian rebel victory could create a Sunni extremist haven in Syria that could inflame sectarian mayhem in his own country as well as in Lebanon and Jordan, the Times said.
All three countries, along with Turkey, are hosts to hundreds of thousands of Syrian refugees, mostly Sunnis.
The killed Syrian soldiers were among some 65 soldiers and Assad regime officials who handed themselves over to Iraqi authorities Friday after Syrian rebels seized the Syrian side of a frontier crossing with Iraq, Baghdad said.
News of the ambush came as Syrian rebel fighters claimed to have gained "near total control" of the contested strategic north-central city of Raqqa, on the north bank of the Euphrates River 100 miles east of Aleppo, after days of heavy clashes.
Rebel videos uploaded to the Internet showed people pulling down and thrashing a statue of Assad's father, Hafez Assad, in Raqqa's central square.
The older Assad was Syria's president from 1971 to 2000.
It was unclear Tuesday if the insurgents retained control of Raqqa. But if they did it would be the first provincial capital completely taken over by the armed resistance.
Syria's military separately launched a new offensive on the western-central cities of Homs and Hama. Anti-Assad activists reported heavy fighting between rebels and regime forces backed by tanks and warplanes.
In Saudi Arabia U.S. Secretary of State John Kerry met with his Saudi counterpart, then said, "The United States will continue to work with our friends as we did in Rome to empower the Syrian opposition to be able to hopefully bring about a peaceful resolution, but if not, to continue to put pressure on Bashar Assad."