The Syrian government is responsible for more than 2,000 deaths in its crackdown against protests, says US Secretary of State Hillary Clinton.
She spoke as an army assault against protest hub Hama was reported to have killed dozens of people in recent days.
Residents of the city say snipers and tanks are firing on civilians and food and medicine are running low.
Activists have dismissed a government decree to allow opposition parties after decades of Baath party rule.
Multi-party rule was a key demand of protesters who have been taking to the streets in large numbers across Syria since mid-March to call for the overthrow of President Bashar al-Assad.
Mrs Clinton repeated an earlier statement that the United States believed Mr Assad had lost legitimacy in Syria.
"We've seen the Assad regime continue and intensify its assault against its own people this week," she said on Thursday.
"We think to date the government is responsible for the deaths of more than 2,000 people of all ages."
She added that the US and its allies were working to apply more pressure on Syria beyond the addition of more individuals to a sanctions blacklist.
Human rights have estimated that more than 1,600 civilians have been killed since anti-government protests began in March.
At least 150 people have been killed since Sunday, mainly in Hama, the rights groups say, as the military intensifies its efforts to quell dissent.
Mr Assad blames the current violence on "armed criminal gangs" backed by unspecified foreign powers.
International criticism of Syria has been mounting since the UN Security Council adopted a statement on Wednesday condemning the government of President Assad for "widespread violations of human rights and the use of force against civilians".
President Dmitry Medvedev of Russia, long an ally of Syria, said Mr Assad would "face a sad fate" unless he urgently carried out reforms and reconciled with the opposition.
And EU states extended their sanctions against Syria, adding more names to a list including President Assad and 34 other people as well as firms linked to the military. They stopped short of targeting the oil industry and banks, however.
Dozens of people are believed to have been killed in a five-day military assault on Hama, with residents saying on Thursday that tanks had shot their way into Assi (Orontes) Square, in the centre of the city of 800,000 people.
Activists said as many as 30 more people were killed in Hama late on Wednesday, after Ramadan prayers.
Communication with the city is all but completely cut off, as are water and electricity, correspondents say.
One resident who escaped the city on Wednesday told the BBC it looked "exactly like a battlefield... like a Gaza Strip kind of city. Like some villages in Iraq when the US army invaded it. That's how it looks like".
He said artillery was firing at buildings and snipers were shooting at anyone they saw on the streets.
Many people had left the city, he said, but for those left, food and medicine were running low.
Another resident said "people are being slaughtered like sheep while walking in the street.
"I saw with my own eyes one young boy on a motorcycle who was carrying vegetables being run over by a tank," the man told Associated Press news agency.