The United States closed its embassy in Damascus on Monday amid escalating violence in Syria and an increasingly urgent effort by the Obama administration and like-minded governments to raise pressure on President Bashar al-Assad to leave power.

After the failure of an anti-Assad resolution at the U.N. Security Council on Saturday, French President Nicolas Sarkozy and German Chancellor Angela Merkel echoed a U.S. call to form an international political coalition, similar to the one that preceded military intervention in Libya, to coordinate anti-Assad actions.

But even as shelling continued Monday in the city of Homs, the Western allies made clear that they had no plans for military action in Syria. Instead, U.S. and other officials spoke of tightening economic sanctions in the hope of strangling Assad’s government and persuading those around him to abandon him, as they look for ways to help the weak and unorganized Syrian opposition.

Russian Foreign Minister Sergei Lavrov was scheduled to visit Damascus on Tuesday for talks with Syrian officials, but there was little optimism that the meetings would alleviate the crisis. Along with China, Russia vetoed the

anti-Assad resolution, and on Monday, Lavrov denounced “certain Western states” for what he called their “hysterical statements” about the veto.

The U.N. resolution’s proponents viewed the veto as a “Cold War curtain call” by Russia to protect its sole remaining ally in the Middle East against what it denounced as Western intervention, said a senior Arab diplomat.

“The big goal now is to figure out what people can do together outside the United Nations,” said the diplomat, speaking candidly on the condition of anonymity. “The issues themselves are going to be the same — an arms embargo, perhaps sanctions on individual bank accounts, travel bans. I don’t think there are any more, newer ideas.”

The Gulf Cooperation Council, a group of Persian Gulf countries that drew up a transition plan for Yemen, has scheduled a meeting for Saturday to consider next steps on Syria. The Arab League will convene Sunday in Cairo.

Although a cooperative, Libya-like alliance could improve the flow of supplies into opposition strongholds, analysts said they do not expect to see Syria’s scattered resistance movement coalesce quickly into a fighting force.

“There’s a scramble now for options,” said Michael Singh, who was senior director for Middle East affairs at the National Security Council in the George W. Bush administration. On one hand, arming the rebels would almost certainly “fuel the fighting and accelerate the descent into civil war,” Singh said. But doing nothing, he said, could make the inevitable conflict longer, bloodier and more dangerous.

Others noted that Assad still has a lifeline to Iran and Russia, as well as the potential for paramilitary support from groups such as Hezbollah and Hamas.

“For Obama, it highlights the painful reality that, despite his tough ‘Assad must go’ rhetoric, he’s neither willing nor able to do much more,” said Aaron David Miller, an adviser on the region to Democratic and Republic administrations and currently a public policy scholar at the Woodrow Wilson Institute.

“Syria will be a tragic slow bleed with no good options or outcomes,” Miller said.

On Monday, residents and reporters in Homs, where hundreds were reported killed over the weekend, said heavy artillery was falling, particularly in the opposition stronghold of Baba Amr.

Opposition groups put Monday’s death toll at about 40, as residential neighborhoods were bombarded; also hit was a field hospital where dozens of injured were being treated because, according to activists, access to government hospitals was blocked.

Video footage from Homs showed severe damage to homes and mosques, numerous gruesome injuries and mangled bodies, and repeated heavy artillery fire in different areas of the city.

Col. Malik Kurdi, a spokesman for the armed opposition force known as the Free Syrian Army, said its fighters still controlled a number of areas in the city, although he declined to say how many men were operating there. Kurdi said the group conducts mainly defensive operations, although it sometimes attacks security forces to take their weapons and for “protection.”

“The regime’s army will not be able to take over the area,” he said, “because we have the support of the people and because the Free Syrian Army has the utmost determination and will.”

Syrian state media reported that violence by “armed terrorists” had increased across the country, and that terrorists had shelled Homs neighborhoods and attacked security officers, provoking clashes, while two oil pipelines near the city had been bombed.

The military has also escalated operations near the capital. The government continued its aggressive campaign to retake Zabadani, a town 20 miles outside Damascus where opposition fighters last month negotiated a withdrawal of troops.

The decision to shutter the U.S. Embassy and withdraw the remaining diplomats followed growing administration concern about the security of the mission as the revolt, and the government crackdown, have edged closer to the capital in recent weeks. The embassy building, State Department spokeswoman Victoria Nuland said Monday, “is right at the confluence of many main streets. It has no protection or setback to speak of.”

Diplomats had appealed to Syrian authorities for permission to bring in extra staff and take further measures to secure the embassy, but they met with resistance.

Nuland said U.S. Ambassador Robert S. Ford and all diplomats and American citizens associated with the embassy left the country Monday morning after Ford informed Syrian Foreign Minister Walid al-Moualem in a personal visit. The State Department issued a travel warning recommending that all U.S. citizens in Syria “depart immediately.” It said the Polish Embassy will now “serve as the protecting power for U.S. interests” in Syria.

Nuland said that the United States has “not broken diplomatic relations with Syria” and that Ford will continue to operate from the State Department, where he will “maintain contact with the Syrian people.”

Ford, who led U.S. criticism of the Syrian regime in the early months of the uprising, was called home from Damascus in October out of concern for his safety but returned to Syria in early December. His meetings with activists and his vivid Facebook postings have drawn the ire of the Damascus government.

Britain summoned its ambassador from Damascus to London for consultations, although a Foreign Office official said he will probably return.

But in a lengthy statement to Parliament, Foreign Secretary William Hague condemned the Russian veto and Syria’s increasingly bloody crackdown. “This is an utterly unacceptable situation which demands a united international response,” he said.

DeYoung reported from Washington. Staff writer Joby Warrick contributed to this report.