White House spokesman Jay Carney said that President Bashar al-Assad was leading his country along a "dangerous path".
Activists claimed that 25 protesters were shot dead by security forces as they poured onto the streets of towns and cities across the country following Friday prayers. They said 11 of the deaths occurred in Maaret al-Numan in Idlib province after forces opened fire on a large crowd.
A London-based Syrian group said that nine people were also killed in the port of Latakia - and another two in southern Daraa province. Three people were also reportedly killed in the Qabun area of the capital Damascus.
Army units moved on the northwestern town of Jisr al-Shughur on Friday but activists say most of the town's 50,000 inhabitants had fled, many to Turkey.
Close to 3,000 Syrian refugees are now sheltering in Turkey, which this week described the crackdown on Syrian civilians as "an atrocity",
Diplomats held fresh talks on Friday on a European-proposed resolution condemning Syria’s deadly crackdown but did not get close to a full vote. Talks are set to extend throughout the weekend.
And the Syrian President is refusing to take telephone calls from UN chief Ban Ki-moon, as the UN Security Council headed into a weekend of talks on a resolution to condemn his crackdown.
Ban tried to call Assad on Thursday but was told the Syrian leader was "not available," said UN spokesman Martin Nesirky.