Although the diplomatic effort was stymied at the U.N. by vetoes from Russia and China, the moves by the U.S. and Britain were a clear message that Western powers see no point in engaging with Assad and now will seek to bolster Syria's opposition.
"This is a doomed regime as well as a murdering regime," British Foreign Secretary William Hague told lawmakers as he recalled his country's ambassador from Syria. "There is no way it can recover its credibility internationally."
President Barack Obama said the Syrian leader's departure is only a matter of time.
"We have been relentless in sending a message that it is time for Assad to go," Obama said during an interview with NBC. "This is not going to be a matter of if, it's going to be a matter of when."
The most serious violence Monday was reported in Homs, where Syrian government forces, using tanks and machine guns, shelled a makeshift medical clinic and residential areas on the third day of a relentless assault, killing a reported 40 people, activists said. More than a dozen others were reported killed elsewhere.
Those deaths followed a regime onslaught in Homs that began Saturday, the same day Syria's allies in Russia and China vetoed a Western- and Arab-backed resolution aimed at trying to end the crackdown on dissent. Some 200 people died, the highest death toll reported for a single day in the uprising, according to several activist groups.
Even as the U.S. steps up pressure on Assad to halt the violence and relinquish power, Obama said a negotiated solution was possible, without recourse to outside military intervention.
Later, however, White House spokesman Jay Carney said the administration was taking "no options off the table."
In a signal that the window for diplomatic efforts may at some point close, Carney said: "We need to act to allow a peaceful political transition to go forward before the regime's escalating violence puts a political solution out of reach."
U.S. Ambassador Robert Ford and 17 other U.S. officials left Syria on Monday, arriving in Amman, Jordan, several hours later, before traveling back to the United States.
As part of what was clearly a concerted Western effort, the Italian Foreign Ministry said it had also summoned Syria's ambassador in Rome to express "the strongest condemnation ... over the unacceptable acts of violence perpetrated by the regime of Damascus against the civilian population."
More than 5,400 people have been killed in Syria since the uprising began in March, the U.N. said early last month. Hundreds more are believe to have been killed since then, but the U.N. says the chaos in the country has made it impossible to cross-check the figures.
Syria has blocked access to trouble spots and prevented independent reporting, making it nearly impossible to verify accounts from either side. The Assad regime says terrorists acting out a foreign conspiracy to destabilize the country are behind the uprising, not people seeking to transform the authoritarian regime.
There are fears that international intervention, akin to the NATO airstrikes that helped topple Libya's Gadhafi, could make the already combustible conflict in Syria even worse.
Syria is a highly unpredictable country, in part because of its web of allegiances to powerful forces, including Lebanon's Hezbollah and close ally Iran.
The country also has multiple sectarian divisions, which the uprising has laid bare. Most of Syria's 22 million people are Sunni Muslim, but Assad and the ruling elite belong to the minority Alawite sect - something that has bred seething resentments.
The violence has reinforced opposition fears that Assad will unleash even greater firepower to crush dissent now that protection from China and Russia against any U.N.-sanctioned action appears assured.
After the U.N. veto, the commander of the rebel Free Syrian Army, Col. Riad al-Asaad, said "there is no other road" except military action to topple Assad.
With diplomacy at an impasse, Secretary of State Hillary Rodham Clinton has called for "friends of democratic Syria" to unite and rally against Assad's regime, previewing the possible formation of a group of like-minded nations to coordinate assistance to the Syrian opposition.
Expanding on the idea Monday, State Department spokeswoman Victoria Nuland said the U.S. would seek to work outside the U.N. to "strengthen and deepen and broaden the international community pressure on Assad, ... to work with as many countries as we can to increase both regional sanctions and unilateral national sanctions on the Assad regime."
The contact group is likely to be similar, but not identical, to the one established last year for Libya, which oversaw the international help for Gadhafi's opponents. It also coordinated NATO military operations to protect Libyan civilians, something that is not envisioned in Syria.
Syria has seen one of the bloodiest crackdowns since the wave of Arab uprisings began more than a year ago. Deaths in Egypt, Tunisia and Yemen have numbered in the hundreds. Libya's toll is unknown and likely higher than Syria's, but the conflict differed there: Early on, it became an outright civil war between two armed sides.
Syria, in contrast, has developed into a murderous grind - although many fear it is swiftly developing into a civil war. Though internationally isolated, Assad appears to have a firm grip on power with the loyalty of most of the armed forces, which have moved from city to city to put down uprisings.
In each place, however, protests have resumed, and now army defectors and others are taking up arms to fight back, adding to the bloodshed.
The Obama administration has long called on Assad to leave power, and officials insist his regime's demise is inevitable.
However, just over a year ago, the administration sought to engage Damascus and sent Ford to the country in the hopes of prying away Iran's main ally in the Arab world and gaining a more willing partner in American efforts to forge stability in Lebanon and peace among Israel and its Arab neighbors.
Syria had gone years without an American ambassador after the Bush administration broke ties over Syria's alleged role in the 2005 assassination of politician Rafik Hariri in neighboring Lebanon, and it remains on a U.S. "state sponsor of terrorism" list.
Assad largely shrugged off U.S. attempts to pull his nation away from its alliances with Iran, Hamas and Hezbollah. And as protests escalated in Syria, Ford took on an increasingly high-profile role defending the rights of Syrian protesters. Threats led the U.S. to pull him out of the country in October, but he returned in December to what officials described as an important job monitoring abuses and developments on the ground.
Shutting the U.S. embassy and recalling American personnel is short of a complete break of diplomatic relations. A year ago, the U.S. similarly closed its mission in Libya's capital as violence escalated, yet maintained for some time communications with senior Libyan officials as it sought to convince Gadhafi loyalists to abandon the regime and others to advance possible surrender scenarios.
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Klapper reported from Washington.
