Syria was under growing pressure on Thursday to stop using military force against pro-democracy protesters after Washington imposed sanctions on President Bashar al-Assad for rights violations.
Syrian security forces have cracked down on flashpoints of unrest with tanks, gunfire and mass arrests in an attempt to crush the revolt against four decades of rule by the Assad family.
Western powers at first made only muted criticisms of Assad's handling of two months of unrest, then stepped up their condemnation and imposed sanctions on key Syrian figures.
Washington's decision to target Assad specifically raises the stakes in a conflict human rights groups say has cost the lives of at least 700 civilians and poses questions about whether the West may ultimately seek his overthrow.
A senior U.S. official said the new sanctions were meant to force Assad to carry out promised political reforms.
"President Assad has a clear choice: either to lead this transition to democracy or to leave," the official, speaking on condition of anonymity, told reporters.
Leading Syrian opposition figure Haitham al-Maleh said the decision meant "members of the regime are now under siege".
"Any move by the international community may help the Syrian people in continuing their uprising," he told Reuters.
The U.S. Treasury Department said it would freeze any assets owned by Syrian officials in the United States or which fall within U.S. jurisdiction, and bar U.S. individuals and companies from dealing with them.
The sanctions also include Syria's vice president, prime minister, interior and defence ministers, the head of military intelligence and director of the political security branch.
Treasury official David S. Cohen said the move sent a clear message to Assad and his inner circle that "they will be held accountable for the ongoing violence and repression", but it was unclear what assets, if any, would be blocked.
An EU diplomat said the European Union was expected to extend sanctions on Syria next week to include Assad as well. "What I detect from members states...is that there is a clear majority, if not now a consensus, for putting him on the list," the diplomat told Reuters.
The unrest in Syria began when protesters, inspired by uprisings in other parts of the Arab world, took to the streets in March calling for greater freedoms and an end to corruption.
As Assad made some token gestures towards reforms, including lifting a decades-old emergency law, the crackdown by troops, security forces and irregular Assad loyalists intensified.
Syrian authorities blame most of the violence on armed groups backed by Islamists and outside powers who they say have killed more than 120 soldiers and police.
Switzerland said it would impose travel bans on 13 top Syrian officials not including Assad, and freeze any of their assets held in Swiss banks, matching a decision by the EU.
Amnesty International welcomed Washington's move and called on U.S. President Barack Obama, who is expected to give a speech on Thursday about the Arab uprisings, to impose an arms embargo.
"The Syrian government has responded to peaceful calls for reform with the most brutal tactics clearly intended to shatter the will of the people," said Amnesty's T. Kumar.
"President al-Assad and those around him must be held individually criminally responsible before the ICC (International Criminal Court) or national courts of states exercising universal jurisdiction."
Syria has barred most international media from operating in the country, making it hard to verify reports from activists and officials.
On Saturday, Syrian troops went into Tel Kelakh -- a day after a demonstration there demanded "the overthrow of the regime", the slogan of revolutions that toppled Arab leaders in Egypt and Tunisia and challenged others across the Middle East.
"We're still without water, electricity or communications," a resident of Tel Kelakh said by satellite phone, as tanks shelled the town for a fourth day on Wednesday.