The United States is considering targeted sanctions against Syrian officials to respond to "completely deplorable" violence used by Damascus's forces to crush dissent, an official said on Monday.
Signs of a more muscular US response to violence in Syria followed an assault by Syrian troops backed by tanks in the flashpoint town of Daraa, which killed at least 25 people, as a building crackdown reached new heights.
"The brutal violence used by the government of Syria against its people is completely deplorable and we condemn it in the strongest possible terms," said National Security Council spokesman Tommy Vietor.
"The United States is pursuing a range of possible policy options, including targeted sanctions, to respond to the crackdown and make clear that this behavior is unacceptable.
"The Syrian people's call for freedom of expression, association, peaceful assembly, and the ability to freely choose their leaders must be heard."
Washington has issued repeated statements by senior officials including President Barack Obama calling for an end to violence and political reform in Syria, but faced domestic criticism for not taking more concrete steps.
But Monday's crackdown appeared to mark a point at which the administration - which has sought to engage Syria as a key regional power player - had little choice but to be seen to act more robustly.
As well as the Daraa crackdown, Syrian troops also on Monday launched assaults on the Damascus suburbs of Douma and Al-Maadamiyeh, witnesses said.
Washington has come under increasing pressure for more biting action against the government of Bashar al-Assad, as protests widen in Syria and pose another challenge to the coherence of US policy as unrest rocks the Middle East.
The Wall Street Journal earlier reported that the White House was preparing a presidential executive order freezing assets of senior Syrian officials and banning them from any business deals in the United States.
The measures would have a strong symbolic element but the Journal reported that they would not have much impact on Assad's inner circle as few regime kingpins have substantial holdings in the United States.
But should similar measures be adopted by Europe, they could have more bite, given more substantial holdings in the continent by the Assad family, the paper said, adding the US move could pressure European governments for action.
So far, Washington has not threatened to recall its US ambassador to Syria, a post filled in January after a six-year absence, as Obama sought to court Damascus as part of a broader Middle East diplomatic push.
The crackdown in Syria poses a stiff dilemma for the Obama administration, which has found its regional policy repeatedly challenged by the wave of unrest in the Middle East.
On the one hand, Washington could stand to profit from a fall of Assad's minority Allawite regime, which is allied to Shiite Iran, a longtime US foe, and which wields power detrimental to US goals in Lebanon.
On Friday, Obama accused Syria of blaming outsiders for its troubles, and specifically said it was seeking Iranian help to suppress its citizens.
But though it may welcome a weakening of Syrian ties to Iran, Washington also appears concerned about the uncertainty of what could follow a fall of the Assad regime amid fears of an even more radical government.
The United States had appeared to hope that eventual Syrian talks with Israel could help pave the way for a future Middle East peace compact and that Assad could be coaxed towards reform and dialogue.
Rights activists said a 3,000-strong military force swooped down on Daraa at dawn on Monday, with tanks taking up position in the town centre and snipers deploying on rooftops.
Some 390 people have been killed in security crackdowns since protests erupted in mid-March, according to rights activists and witnesses.
The latest bloodshed came despite Assad's decision on Thursday to end a draconian state of emergency, imposed by the Baath party when it seized power in 1963.
He also abolished the state security court that has tried scores of regime opponents outside the normal judicial system and issued a decree "to regulate" peaceful demonstrations.