"The American forces from Iraq committed cold-blooded murder," the government newspaper Tishrin wrote, the Agence France-Presse reported. "They committed a war crime in killing eight Syrian civilians in a quiet village."
Syria said U.S. soldiers attacked the village from helicopters and stormed a building.
The U.S. military in Iraq said in a statement it did not have "any information" on the incident.
The U.S. government has neither confirmed nor denied the incident, but an unnamed U.S. military official in Washington confirmed Sunday that special forces had conducted a raid in Syria that targeted foreign fighters, the Associated Press reported.
"We are taking matters into our own hands," the official said.
If confirmed, the attack would be the first such U.S. military strike inside Syria since the 2003 invasion of Iraq.
"I will say this once and you can ask me as many times as you want to: I have no comment on any alleged operation in Syria," Pentagon spokesman Bryan Whitman said, according to Reuters.
The U.S. military has increased security on the border in recent months to prevent fighters and weapons from coming into Iraq from Syria, according to the Washington Post.
"This is an outrageous raid which is against international law," Syrian Ambassador to London Sami al-Khiyami said, reported Reuters. "It is a terrible crime. I don't know the political meaning of it. We are expecting clarifications from the Americans."
Iraqi government spokesman Ali al-Dabbagh said the targeted area was used by insurgents to launch attacks on Iraq.
"The latest of these groups ... killed 13 police recruits in an (Iraqi) border village. Iraq had asked Syria to hand over this group which uses Syria as a base for its terrorist activities," al-Dabbagh told Reuters.
"We are trying to contain the fallout from the incident," Iraqi Foreign Ministry undersecretary Labid Abbawi said, reported the AP. "It is regrettable and we are sorry it happened."
Syria on Sunday summoned the top U.S. diplomat in the country, Maura Connelly, to notify her of "Syria's condemnation and complaint of this dangerous aggression," the Syrian Arab News Agency reported.
The dead were described as all civilians, a father and his four children, a couple and another man. Syrian state television broadcast pictures of the scene, and showed the dead bodies.
Confirmed U.S. attacks in foreign territory are rare. The Syrian raid appears to echo one taken recently in America's other war. In July, President Bush secretly approved U.S. military raids inside anti-terror ally Pakistan, which has been unwilling or unable to stem the flow of militants hiding in Pakistan and waging cross-border raids into Afghanistan, according to the AP. Only one such raid has been carried out, a senior Pakistani government official said.
Helicopter-borne U.S. special forces conducted a raid Sept. 3 inside Pakistan - the only one known so far following Bush's order. Islamabad has complained bitterly about the move, which it says killed some two dozen people, including civilians, and violated its sovereignty.
"Now neighboring countries have a good reason to be concerned about the continued U.S. presence in Iraq," prominent Kurdish politician Mahmoud Othman told the AP.