Tens of thousands of Syrians surged onto the streets to stage anti-government demonstrations in defiance of an escalating military crackdown Friday, amid signs that the international community is still unwilling to call directly for President Bashar al-Assad’s departure.

Human rights groups said at least 15 protesters were killed by security forces as the Syrian government defied international pressure to halt the violence and enact reforms.

Among the towns and cities where residents turned out for protests after noontime prayers were Hama and Deir al-Zour, the two main targets of the crackdown, suggesting that the government’s attempt to suppress the five-month-old uprising by means of overwhelming force is not scaring people into staying home.

In one sign of the growing frustration of the protesters, who have taken to the streets for 22 consecutive Fridays, chants at this week’s demonstrations called not only for the toppling of the government but the death of Assad. “The people want the execution of the president,” crowds roared in demonstrations in Damascus, Daraa, Hama, Homs and other cities and towns, activists said.

At a news conference in Washington, Secretary of State Hillary Rodham Clinton repeated the U.S. demands to Assad to “immediately stop the violence, withdraw your security forces, respond to the legitimate aspirations of the Syrian people for a democratic transition in concrete and meaningful ways.” She also urged European countries to impose sanctions on Syria’s oil and gas industry.

But an announcement by Turkish Prime Minister Recep Tayyip Erdogan on Wednesday giving Assad 10 to 15 days to implement reforms seemed to have slowed the momentum that had been building toward calls for his departure.

Syrian activists said Erdogan’s deadline was widely interpreted as a green light to Assad to continue the crackdown, in which tanks have been dispatched into key protest cities to bombard them into submission. Hundreds of people have been killed and thousands detained since the launch of an intensified offensive on the eve of the Muslim holy month of Ramadan that seemed designed to definitively crush the democracy movement.

“People on the ground are very angry with Turkey,” said a Damascus-based activist, who spoke on the condition of anonymity because he fears for his safety. “The talk on the street is that Turkey has a secret agreement with Assad.”

Earlier in the week, U.S. officials said President Obama would soon call for Assad to leave, a step that many opposition activists hope would further undermine the regime.

But Clinton made it clear Friday that the United States does not want to proceed with such a call without first securing the consensus of key allies, “so that there will not be any temptation on the part of anyone inside the Assad regime to claim that it’s only the United States or maybe it’s only the West.”

A key concern, she explained in an interview with CBS on Thursday, is that the largely ad hoc protest movement has still not coalesced around an identifiable leader or plan for a post-Assad Syria.

“The pressure requires an organized opposition. And there isn’t one,” she said. “We certainly think Syria deserves democracy, but we also know that you have to replace somebody with somebody else. And that somebody else is still in formation.”

Syria’s stability is of vital concern to neighboring Turkey, which has cultivated a close relationship with Assad in recent years with a view to broadening its influence across the Arab world. Key Turkish trade routes to the countries of the Persian Gulf pass through Syria, and there are widespread fears that Assad’s ouster would trigger civil war.

On Friday, Turkey’s state-run Anatolian news agency published a letter by Turkish President Abdullah Gul appealing to Assad to act.

“I don’t want to see you looking back one day and regretting that what you have done was too little and too late,” the letter said. “Leading the change instead of being carried away by the winds of change will place [you] in a historical position.”

But the scale of the recent protests, which are now being held daily, suggests that the government will be unable either to quell the dissent by force or to enact reforms that will permit it to survive, locking the country in a cycle of continuing protests and bloodshed.

“The regime feels it has no choice but to continue with the crackdown because if it pulls back the troops, huge numbers will pour into the streets,” said Wissam Tarif, a human rights activist with the global advocacy group Avaaz. “What we’re seeing is a regime that is very determined and protesters who are very determined.”

An activist contacted by satellite telephone in Hama said protests were staged at 14 mosques around the city Friday despite the ubiquitous presence of the security forces. One demonstrator was killed and 10 were injured when troops opened fire, he said.

“If we don’t keep protesting, they will finish us all,” he said. “We will never give up.”

The largest number of deaths Friday occurred in the suburbs of Damascus, where at least seven people were killed, and the rest were scattered among Homs, Hama, Aleppo and Idlib, according to Avaaz.

Staff writers Joby Warrick and Mary Beth Sheridan in Washington contributed to this report.