The White House has said yesterday that touse here is no sign of Syria abiding by an international plan to bring fighting to an end hours before a UN deadline.
Under the peace deal, Syrian troops must withdraw from protest centres today but US officials said the situation appeared to have worsened.
Yesterday proved to be one of the bloodiest days of the uprising, with activists reporting more than 100 dead.
Earlier violence spilled across Syria's borders with Turkey and Lebanon.
Turkey protested angrily, summoning Syria's envoy in Ankara after two incidents along their joint border left two dead and many injured, including Turkish citizens.
Separately a Lebanese cameraman was shot dead on Lebanon's northern border with Syria.
Meanwhile, UN Secretary General Ban Ki-Moon made a final plea for the Syrian administration to halt all attacks on civilians.