US helicopter-borne troops have carried out a raid inside Syria along the Iraqi border, killing eight people including four children, Syrian officials say.
The official Syrian news agency Sana said the raid took place in the Abu Kamal border area, in eastern Syria.
It said that American soldiers on four helicopters had stormed a building under construction on Sunday night.
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 BBC diplomatic correspondent Jonathan Marcus.
Its timing is curious, coming right 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, our correspondent adds.
Crossing point
"Four American helicopters violated Syrian airspace around 1645 local time [1345 GMT] on Sunday," Sana said.
"American soldiers" emerged from helicopters and "attacked a civilian building under construction and opened fire on workers inside - including the wife of the building guard - leading to [the deaths] of eight civilians", it added.
"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, without giving details of the children's ages.
The village was named as Sukkiraya, 8km (5 miles) 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.
The Iraqi city's mayor, Farhan al-Mahalawi, told Reuters news agency that US helicopters had struck a village on the Syrian side of the border, after which Syrian troops surrounded the site.
Washington has in the past accused Damascus of turning a blind eye to the problem.
