US troops have carried out a raid inside Syria, killing eight people including four children, Syrian officials say.
Syrian media said American soldiers on four helicopters had stormed a building under construction.
It said the raid took place in the Abu Kamal border area, in eastern Syria.
The US says it is investigating. It has previously accused Syria of allowing foreign militants into Iraq.
Syria has summoned the US and Iraqi envoys in Damascus to protest at the raid.
"Syria condemns this aggressive act and holds American forces responsible for this aggression and all of its repercussions," a government official said.
If confirmed, the raid would be the first known attack by US forces inside Syrian territory, says the BBC's correspondent Jonathan Marcus.
Its timing is curious, he adds, coming at the end of the Bush administration's period of office and at a moment when many of America's European allies - like Britain and France - are trying to broaden their ties with Damascus.
The official Syrian news agency Sana reported that American soldiers emerged from helicopters and opened fire on workers inside the building, including the wife of the building guard.
"The helicopters then left Syrian territory towards Iraqi territory," Sana said.
The dead include a man, his four children and a married couple, the Syrian report said.
The village was named as Sukkiraya, 8km from the Iraqi border.
A US military spokesman was unable to confirm or deny the reports, saying it was a "developing situation".
But later the Associated Press news agency quoted an unnamed US military official in Washington as saying that American special forces had attacked foreign fighters linked to al-Qaeda.
"We are taking matters into our own hands," the official said.
The area is near the Iraqi border city of Qaim, a major crossing point for fighters, weapons and money travelling into Iraq to fuel the Sunni insurgency.