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."