Syrians prepared on Saturday to bury at least 12 protesters shot dead by security forces as Washington warned the government's "continued brutality" may delay but will not reverse the process of change.
Rights activists said protests broke out after the main weekly Muslim prayers on Friday as the army pressed its campaign against northern towns and the number of refugees fleeing across the border into Turkey neared 10,000.
A senior US administration official put the toll at around 19 dead.
Rami Abdel Rahman, head of the Syrian Observatory of Human Rights, said five people were killed in the flashpoint central city of Homs, two in the eastern city of Deir Ezzor and two in the Damascus suburb of Harasta.
Two more people were killed in Dael in the restive southern province of Daraa and a 12th person died in the Damascus suburb of Douma, other activists told AFP by telephone.
The United States is weighing whether war crimes charges can be brought against Damascus to pressure the government to end its bloody crackdown on dissent, the US administration official said.
Other measures, including sanctions targeting the country's oil and gas sector, are being considered as part of a broader diplomatic campaign to increase pressure on President Bashar al-Assad.
US Secretary of State Hillary Clinton on Saturday urged a transition to democracy in Syria, saying in a commentary in the Arabic-language Asharq Al-Awsat newspaper that the government's crackdown would not quell the momentum for change.
Clinton wrote under the headline "There Is No Going Back in Syria" that it was "increasingly clear" the crackdown was an irreversible shift in the country's push towards reform, in an English translation provided by the State Department.
The regime's "continued brutality may allow (Assad) to delay the change that is under way in Syria, it will not reverse it," Clinton wrote in the pan-Arab daily published in London.
"The most important question of all -- what does this mean for Syria's future? -- is increasingly clear: There is no going back." Assad's actions have "shattered his claims to be a reformer," Clinton wrote.
A senior administration official said on Friday that the United States was studying whether war crimes charges could be brought against Syria.
Two administration officials, speaking on condition of anonymity, outlined the campaign in a teleconference with reporters, stressing efforts were being made at the United Nations and with regional partners to condemn and isolate the regime.
The official said other measures, including sanctions targeting Syria's oil and gas sector, were being considered as part of a broader diplomatic campaign to increase pressure on Assad.
In Friday's violence, according to Abdel Rahman, "there was intense firing to disperse the demonstrations in (the coastal city) Banias and there were casualties" but the head of the London-based Observatory was unable to give a breakdown.
About 5,000 protesters gathered in Homs, he said, adding demonstrations gripped several other cities and towns including Jableh in the west and in Suweida in the south, where club-wielding forces dispersed hundreds.
Protests also hit Latakia, Maaret al-Nooman and the countryside outside Damascus, activists said.
Witnesses told AFP that a gunman opened fire on a police station in Rikn al-Deen, in Damascus, during a protest, killing a policeman and wounding at least four.
State news agency SANA also reported casualties among the ranks of the security forces. "A member of the security forces was martyred and more than 30 were wounded by gunfire in Homs," the news agency said.
It added that two officers and four members of the security forces were wounded when gunmen attacked a recruitment centre in Deir Ezzor while three policemen were hit by gunfire in the Qabun neighbourhood of Damascus.
The military pressed ahead with its crackdown, sending tanks and troops into the northwestern town of Khan Sheikhun and surrounding villages, according to activists and witnesses.
Nearly 10,000 Syrians have crossed the border into Turkey fleeing the crackdown, an official Turkish source said on Friday.
The Syrian Observatory for Human Rights on Tuesday said the violence has claimed the lives of nearly 1,300 civilians and 340 security force members since it broke out in mid-March.