BEIRUT -- The U.S. closed its Syrian embassy Monday and Britain recalled its ambassador to Damascus in a dramatic escalation of Western pressure on President Bashar Assad to give up power, just days after diplomatic efforts at the United Nations to end the crisis collapsed.

The U.S. evacuated all its diplomats from the country as Syrian forces intensified a shelling assault on the restive city of Homs. The offensive 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 brutal crackdown on dissent.

"We have been relentless in sending a message that it is time for Assad to go," President Barack 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."

Also Monday, British Foreign Secretary William Hague told lawmakers that Britain is using multiple channels to express its "abhorrence" at the violent crackdown, and has summoned Syria's ambassador to the Foreign Office to convey that message.

"This is a doomed regime as well as a murdering regime," Hague said. "There is no way it can recover its credibility internationally."

The onslaught on Homs has reinforced opposition fears that Assad will unleash even greater violence to crush dissent, now that protection from China and Russia against any U.N.-sanctioned action appears assured.

Already, more than 5,400 people have been killed since the Arab Spring-inspired uprising that began in March, according to the U.N.

The decision to close the embassy is the most dramatic U.S. move so far after 11 months of a violent crackdown by Assad's regime.

Even as the U.S. stepped up pressure on Assad to quit, Obama said a negotiated solution in Syria is possible and it should not be resolved by foreign military intervention.

The State Department warned last month it would close the embassy unless Assad's government stepped up its protection. It cited concerns about the safety of personnel and recent car bombs.

In Homs, shells slammed into a makeshift medical clinic and residential areas, killing at least 23 people in the third day of a new offensive on the epicenter of the country's uprising, activists said. Another 10 people were reported killed elsewhere.

In Cairo, Arab League chief Nabil Elaraby said he was "extremely alarmed and concerned" at the use of heavy weapons by regime forces. The League been an important diplomatic force trying to stem the bloodshed, and its proposal for a transition to democracy in Syria was the basis for the U.N. Security Council resolution that Russia and China blocked in a vote Saturday.

The government denied shelling Homs, however, and said "armed terrorist groups" were attacking civilians and police in several neighborhoods. The state-run news agency also said Monday that gunmen killed three soldiers and captured others at a checkpoint in the Jabal al-Zawiyah region of Idlib province, which borders Turkey.

Syria has blocked access to trouble spots in the country and prevented independent reporting, making it nearly impossible to verify accounts from either side as the conflict spirals out of control and turns increasingly violent.

Homs, which many refer to as "the capital of the Syrian revolution," has become a flashpoint of the nearly 11-month-old uprising against Assad. Several neighborhoods in the city, such as Baba Amr, are under the control of rebels.

The threat of both sides turning to greater force increased Saturday when Russia and China vetoed a U.N. Security Council resolution aimed at ending the bloodshed. .

U.N. Ambassador Susan Rice says China and Russia are running the risk of suffering the same sort of international isolation as Assad because of their double veto.

Moscow and Beijing "will come to regret" their votes, Rice told "CBS This Morning."
On Saturday, Syrian forces killed up to 200 people in Homs -- the highest death toll reported for a single day in the uprising -- according to several rights groups. There was no way to independently confirm the toll.

While government forces have in the past used tanks and other weapons, the increased number of victims appear to have resulted from the indiscriminate use of artillery, according to the activists' reports.

"As of 6:30 this morning, the shelling intensified with a rate of one shell every two minutes," Baba Amr activist Omar Sheker said during Monday's bombardment.

The uprising began with mostly peaceful protests against Assad, but government forces responded with a fierce crackdown. Now, army defectors and others are taking up arms to fight back, raising fears of civil war.

China said Monday it was forced to use its veto because the vote was called too soon, before the parties could work out differences in the proposal. But China denied playing spoiler and said it wants to see an end to violence there.

China and Russia have drawn the wrath of the United States, Europe and much of the Arab world for the weekend veto. China says the resolution put undue emphasis on pressuring the Syrian government and prejudged the result of any dialogue between the parties in Syria.

"On the issue of Syria, China is not sheltering anyone nor do we intentionally oppose anyone. We uphold justice and take a responsible attitude," Chinese Foreign Ministry spokesman Liu Weimin said Monday.

Also Monday, an explosion ripped through a gas pipeline in Homs, the state-run news agency, SANA, reported. SANA blamed terrorists. The regime says terrorists acting out a foreign conspiracy are behind the uprising, not protesters seeking change.

The Local Coordination Committees activist group said Monday's shelling in Homs hit a makeshift clinic in Baba Amr, causing casualties.

At least 17 people were killed across the city on Monday, according to the LCC and the Britain-based Syrian Observatory for Human Rights.

Activist Shaker said a paramedic was wounded in the shelling of the clinic and two people who were standing outside died instantly. He added that many volunteers at the hospital were wounded as well as people receiving treatment.

Syria's state-run TV denied government forces were besieging the area, saying activists in the city were setting tires on fire to make it appear as if there was a bombardment.

Syrian security forces are "chasing the terrorists and clashing with them," it said.

On Sunday, the commander of rebel soldiers said force was now the only way to oust Assad, while the regime vowed to press its military crackdown to bring back stability to the country.

"We did not sleep all night," Majd Amer, another activist in Homs, said by telephone. Explosions could be heard in the background. "The regime is committing organized crimes."

Amer said shelling of his neighborhood of Khaldiyeh started at 3 a.m., and most residents living on high floors either fled to shelters or to lower floors. He said electricity was also cut.