At least two rocket-propelled grenades have hit a building of Syria's governing Baath Party in the capital Damascus, residents and activists say.
One witness said security forces had blocked off the square where the office is located, while smoke was seen rising from the building.
If confirmed, it would be the first such attack reported inside the capital since the uprising began in March.
It comes amid growing fears of civil war in Syria, after months of unrest.
The opposition Local Co-ordination Committees said "several" RPG rockets were shot at the Baath Party building in the Mazraa neighbourhood and that two fire brigades have been dispatched to the area.
An unnamed witness told Reuters the attack happened before dawn local time and said the building was mostly empty.
"Security police blocked off the square where the Baath's Damascus branch is located. But I saw smoke rising from the building and fire trucks around it," he said
"It seems to have been intended as a message to the regime," he added.
Brutal crackdown continues
Earlier on Sunday, activists said 14 civilians have been killed as forces loyal to President Bashar al-Assad continued their repression of the opposition.
The Arab League has already suspended Syria, and said if the government didn't stop the crackdown by Sunday morning it would face sanctions.
Syria has agreed in principle to accept the peace plan, but critics accuse it of stalling for time.
In an interview with Britain's Sunday Times, Mr al-Assad was defiant, saying he would "not bow down" in the face of foreign pressure.
The United Nations says more than 3,500 people have died in Syria's repression of the pro-democracy uprising since March, while many others have disappeared or been jailed.
