President Obama on Thursday for the first time explicitly called on Syrian President Bashar al-Assad to step down, a symbolically significant step intended to ratchet up pressure on the government five months after the start of the uprising in that country.

Obama also issued an executive order immediately freezing all assets of the Syrian government subject to U.S. jurisdiction and prohibiting Americans from engaging in any transaction involving the government.

Secretary of State Hillary Rodham Clinton followed President Obama’s statement with a briefing on the U.S. policy shift. (Luis Alvarez/Associated Press)

The move, which follows weeks of pressure on the administration to formally call for Assad to leave office, was coordinated with the European Union and other Western allies. In a joint statement, British Prime Minister David Cameron, French President Nicolas Sarkozy and German Chancellor Angela Merkel called for Assad to step aside, saying he should “face the reality of the complete rejection of his regime by the Syrian people.”

U.S. officials have repeatedly condemned the Syrian crackdown and had already declared that Assad had lost the “legitimacy” to rule. But rights activists and some lawmakers have pressured Obama to go further, as the administration has with regard to other Arab Spring uprisings.

The administration had been reported to have considered calling earlier for Assad to step down. But officials hesitated out of concern that the Syrians could depict such a step as evidence that Assad’s removal was being engineered by Washington.

Just this week, Secretary of State Hillary Rodham Clinton played down the importance of a call for Assad to go.

“It’s not going to be any news if the United States says Assad needs to go,” she said during an appearance at the National Defense University in Washington on Tuesday. “Okay. Fine. What’s next? If Turkey says it, if King Abdullah says it, if other people say it, there is no way the Assad regime can ignore it.”

The administration has already announced several rounds of sanctions targeting Assad and Syria’s intelligence and security establishment. The European Union has also imposed sanctions on Syrian government officials. But all of those measures have so far failed to curb attacks by Syrian forces against protesters.

Among other measures, the latest U.S. sanctions prohibit the import of oil from Syria, a move aimed at depriving Damascus of revenue. That ban, in isolation, is unlikely to be effective because the United States has few economic ties with Syria. But European countries, which have said they are weighing expanded sanctions, are among Syria’s biggest customers for petroleum.

In his statement Thursday, Obama said that Assad’s pledges of reform have “rung hollow while he is imprisoning, torturing, and slaughtering his own people.”

“It is clear that President Assad believes that he can silence the voices of his people by resorting to the repressive tactics of the past,” Obama said. “But he is wrong. As we have learned these last several months, sometimes the way things have been is not the way that they will be. It is time for the Syrian people to determine their own destiny, and we will continue to stand firmly on their side.”

Assad has until now proved resilient in the face of mounting world pressure, and some fear he could intensify the offensive to definitively crush the protests once he realizes he is losing international support.

Activists said on Thursday that there was no evidence the crackdown was easing, despite Assad’s claim in a telephone call with U.N. Secretary General Ban Ki-moon on Wednesday night that troops had been pulled out of towns.

Ninar Said, an activist in Latakia, said three tanks had left the coastal city on Wednesday but that at least 15 remain, along with hundreds of soldiers, security forces and armed civilian government supporters. She said troops opened fire on homes in a Palestinian camp and that security forces were looting and burning shops in another neighborhood.

 The human rights advocacy group Avaaz said that shootings and detentions continued Thursday in several locations across the country and that at least one person was killed.

Still, activists have been clamoring for the United States and other world powers to intensify the pressure on Assad in the hope that some regime supporters and Syrian citizens who remain undecided will be encouraged to support the protesters.

Until now, many Syrians have interpreted the world’s inaction as representing tacit support for the Assad regime’s survival, leaving them reluctant to turn against a government that might ultimately prevail and exact revenge against all those who opposed it.

Liz Sly reported from Beirut.

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