"Every day that goes by, the choice is made by default. He (Assad) has not called an end to the violence against his own people, and he has not engaged seriously in any kind of reform efforts," the chief US diplomat said.
"Every day that goes by, the position of the government becomes less tenable and the demands of the Syrian people for change only grow stronger," Clinton said during a brief press conference.
"And therefore we continue to urge an end to the violence and the commencement of a real process that could lead to the kinds of changes that are called for."
Clinton's deputy spokesman Mark Toner expressed deep skepticism that Assad would follow through with his decree earlier Tuesday of a general amnesty for all political prisoners.
"Obviously, he's said a lot of things in recent weeks and months, but we've seen very little concrete action by President Assad," Toner told reporters.
"He's talked about reform, but we've seen very little in the way of action on that front. I guess we're just waiting to see whether the action comes."
Clinton said she was "very concerned" at the report of a 13-year-old Syrian boy who was allegedly tortured and killed by security forces in the flashpoint Daraa region.
"In fact, I think that what that symbolizes for many Syrians is the total collapse of any effort by the Syrian government to work with and listen to their own people," she said.
"I can only hope that this child did not die in vain but that the Syrian government will end the brutality and begin a transition to real democracy."
The amnesty announcement, which came after two months of deadly anti-regime protests, was shrugged off by Syrian opposition activists gathered in Turkey to discuss democratic change and voice support for the revolt.
The release of political prisoners has been a central demand of protesters who, inspired by uprisings in Tunisia and Egypt, have since March 15 been staging almost daily demonstrations against Assad's autocratic government.
More than 1,100 civilians have been killed and at least 10,000 arrested in a brutal crackdown by the regime on the protests, human rights organizations say.
The European Union imposed targeted sanctions on Assad last week including an assets freeze and travel ban, following similar measures imposed by the United States.
In a May 19 speech on the pro-democracy revolutions sweeping Arab countries, Obama said: "President Assad now has a choice. He can lead that transition or get out of the way.
"The Syrian government must stop shooting demonstrators and allow peaceful protests."
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