The White House has strongly denounced what it calls the Syrian government's "outrageous use of violence" and voiced support for a European-drafted proposed UN Security Council resolution condemning the crackdown and calling for its end.
The White House statement comes after reports that Syrian forces for the first time fired on protesters from helicopter gunships in a northwestern town.
State television said well-armed "terrorist groups" had burned police buildings and killed members of the security forces in Maarat al-Numaan.
The use of the aircraft came on a day of nationwide rallies against President Bashar al-Assad.
Activists and witnesses say at least 32 people were killed on June 10 after government forces opened fire on demonstrators.
The figure could not be independently confirmed because foreign journalists have been banned from reporting inside the country.
Local people said at least 15,000 troops along with some 40 tanks and troop carriers had deployed near Jisr al-Shughour.
Thousands of civilians have fled from the northwestern border area into Turkey, fearing security forces' revenge for incidents in which 120 troops were reported killed this week.
Turkish Prime Minister Tayyip Erdogan said his country would continue to provide a safe haven for Syrians.
"We really cannot close the doors to those people who run away for their lives and flee to Turkey. We have to welcome them, but how long it will last? That is another issue," he said.
In a June 9 television interview, Erdogan broke his silence on the regime's crackdown and described its action toward protesters as "savagery."
At the UN, a spokesman said Secretary-General Ban Ki-moon had been trying to call Assad all week but was told that the president was "not available."
compiled from agency reports