American helicopter-borne troops from Iraq launched an assault on Sunday on a building in a Syrian border village, killing eight civilians, official Syrian media reported.
The government has summoned the official U.S. and Iraqi representatives in protest, state television and the official SANA news agency said.
In Washington, the Pentagon said it had no comment.
"Four American helicopters violated Syrian airspace around 16:45 local time on Sunday. They penetrated eight kilometers into Syria," official Syrian media said.
"American soldiers" who had emerged from helicopters "attacked a civilian building under construction and fired at workmen inside, causing eight deaths," reports said.
SANA named the dead and said they were a father and his four children, a couple and another man.
"The helicopters then left Syrian territory toward Iraqi territory," it said.
The news agency said one person was also wounded in the attack on the village of Al-Sukkiraya, around 550 kilometers northeast of the capital in the Abu Kamal area.
Earlier, the private television channel al-Dunia said nine civilians had been killed in the attack.
The raid appears to have been the first of its type into Syrian territory.
Syria summoned the U.S. and Iraqi envoys to Damascus to protest against what it called a U.S. military attack and to demand that Iraq prevent U.S. forces from "launching aggression against Syria" from its territory, official media said.
"Syria condemns and denounces this act of aggression and U.S. forces will bear the responsibility for any consequences," SANA quoted an unidentified official as saying.
"Syria also demands that the Iraqi government accept its responsibilities and launches an immediate inquiry following this dangerous violation and forbids the use of Iraqi territory to launch attacks on Syria," it said.
"We are in the process of investigating this" reported attack, Sergeant Brooke Murphy, a U.S. military spokeswoman, said in Baghdad.
The Iraqi defense ministry refused to comment, on the grounds the incident took place inside Syria.
U.S. commanders say Syria is the main transit point for foreign jihadists crossing into Iraq. Washington has blamed Damascus for turning a blind eye to the problem.
On Oct 16, Iraqi forces arrested seven Syrian "terrorist" suspects at a checkpoint near the city of Baquba, a hub of al-Qaida fighters, the Baghdad defense ministry said.
Iraqi President Jalal Talabani told U.S. President George W Bush last month that Iran and Syria -- long targets of U.S. blame over the deadly unrest in Iraq -- no longer pose a problem.
Iraqi officials have also said that Syria has been boosting border security.
Syria's first ambassador to Iraq in 26 years took up his post in Baghdad this month, marking the official end of more than two decades of icy relations.
On Sept 28, U.S. Secretary of State Condoleezza Rice confirmed she had met her Syrian counterpart, Walid Muallem, to discuss Middle East peace efforts despite renewed criticism from Washington over Syrian policies.
Their talks came after Bush slammed Syria in his farewell address to the U.N. General Assembly. "A few nations -- regimes like Syria and Iran -- continue to sponsor terror," he charged.
Washington has also accused Damascus of failing to give adequate cooperation to the International Atomic Energy Agency in its investigation into a mystery facility bombed by Israel in September last year that U.S. officials have charged was a nuclear plant.© Wire reports