The Syrian government called the deadly raid an act of "criminal and terrorist aggression," but the Iraqi government said "anti-Iraq terror" activity had taken place in this area just across the border on the Euphrates River.
It came just days after the commander of U.S. forces in western Iraq called the Syrian border an "uncontrolled" gateway for fighters entering Iraq and efforts were being stepped up to secure it.
The raid appeared to take the Baghdad government by surprise and intensified worries that it could be used as ammunition for critics of the troubled U.S.-Iraqi security deal.
Syria said four U.S. military helicopters attacked a civilian building under construction just after sundown Sunday about five miles from the Iraqi border just outside Sukkariyeh.
A statement by the government said eight people were killed. However, local officials said seven men were killed and two others wounded, including a woman among the injured. An Associated Press journalist at Monday's funerals in the village's cemetery saw the bodies of seven men, which family members later buried.
A U.S. military official in Washington confirmed Sunday that special forces had conducted a raid in Syria that targeted the network of al Qaeda-linked foreign fighters moving into Iraq. The American military in Baghdad said it did not have any information.
Amateur video taken by a villager on a mobile phone at the scene showed four helicopters flying toward the site as villagers point to the skies in alarm. An Associated Press journalist at the attack site in far eastern Syria saw the grainy video Monday. It didn't show the helicopters landing.
Another villager told the AP he saw at least two men at the scene taken into custody by U.S. forces, and whisked away by helicopter. The villager spoke on condition of anonymity because he feared for his life.
In London, Syria's Foreign Minister Walid al-Moallem offered harsh words for Washington.
"This to us is cowboy politics," al-Moallem told reporters after meeting his British counterpart. "I hope it doesn't come to a confrontation, but if that's what they want, then we'll be ready."
Iran also condemned the attack as did Russia, which has had close ties with Syria since Soviet times.
Iraqi officials said they had no advance warning on the raid, and the government responded carefully to the aftermath, seeking to contain possible diplomatic damage with Syria while at the same time not offending the U.S.
Chief spokesman Ali al-Dabbagh said Iraq was seeking good relations with Syria but added that Baghdad had asked the Syrians in the past to "hand over terror groups operating on Syrian territory."
He also noted that the attack occurred in an area where "anti-Iraq terror activity" had taken place.
"We cannot judge this operation at the moment. Our information comes from the media for the time being, and there maybe some confusion there," he said. "We must wait for our investigation to finish. We are in touch with the American side and we expect them to hand us a report on the raid."
Those concerns mounted after insurgents ambushed a minibus carrying Iraqi police recruits last May, killing 11 of them in an attack about 20 miles from the Syrian border.
American special operations troops have been working for months trying to shut down Sunni extremist networks that smuggle weapons and fighters through the Jazira desert of northern Iraq to Mosul, were al Qaeda and other Sunni militants remain active.
But the timing of the raid raised concerns it could hurt the uncertain U.S.-Iraq security agreement.
Syria and its regional ally Iran have expressed concern that a long-term U.S. presence could allow the Americans to use Iraq as a staging ground for attacks against its neighbors.