The United States came down heavily on Syria saying that President Bashar al-Assad's regime has lost legitimacy and that the rulers were accountable for the deaths of more than 2,000 pro-democracy protesters.

"We are working around the clock to try to gather up as much international support for strong actions against the Syrian regime as possible. I come from the school that actions speak louder than words," Clinton told the media on Thursday.

She repeated that Assad has "lost his legitimacy to govern the Syrian people."

Meanwhile, a Thursday night security crackdown on pro-democracy protesters near capital Damascus and southern Syria left at least four dead, reports said. According to the Syrian Revolution Coordinating Committee, at least three died when security forces fired at demonstrators at the town of Derra, the nerve center of the anti-government protests. Another demonstrator was killed in Qadam, near Damascus.

Earlier, a Jerusalem Post report said at least 45 civilians were killed in a tank assault by Syrian troops at the besieged city of Hama, citing activist sources. The sharp escalation in government repression of demonstrations came a day after the United Nations condemned the regime’s use of violence on demonstrators.

Hama was the scene of a violent crackdown of anti-regime mobs in 1982 when Assad's father crushed Islamist protests, causing the death of several hundred people. "Hama has been cut off. We're in the dark and of course we're very worried," said a Human Rights Watch researcher, according to the Post.

On Tuesday, the U.S. envoy to Syria, Robert Ford, told the Senate the regime was using "constant brutality" and "atrocious torture" against unarmed civilians.

Clinton has repeated the U.S. stance that the regime has killed more than 2,000 protesters. "We've seen the Assad regime continue and intensify its assault against its own people this week ... We think to date the government is responsible for the deaths of more than 2,000 people of all ages," Clinton said.

Wednesday's UN resolution said Assad's government committed "widespread violations of human rights and the use of force against civilians". The U.S. and the European Union also deiced to toughen sanctions against Damascus.