BEIRUT — Syria’s neighbors have turned decisively against President Bashar Assad, launching a diplomatic campaign against his crackdown on the country’s pro-democracy movement that analysts say could have a major impact on important pillars of Assad’s support.

Even as Syrian armed forces pushed against several opposition strongholds Monday, international action against the government mushroomed. The diplomatic pressure marked a significant change from the world’s cautious response over most of the past five months.

Western countries so far have spearheaded efforts to stop the violent crackdown, including a U.N. Security Council statement last week that condemned the offensive. But the Arab League and the Gulf Cooperation Council, a body of six wealthy Arabian peninsula kingdoms, also denounced it over the weekend. Saudi Arabia, Kuwait and Bahrain withdrew their ambassadors on Monday.

In addition to those Arab countries, Turkey was preparing to harden its approach. Reports in Turkey, once a steadfast ally, said Foreign Minister Ahmet Davotoglu would arrive in the Syrian capital Damascus on Tuesday to deliver an ultimatum.

“(Turkey and Syria) will sit down and talk for one last time,” the Turkish daily Hurriyet quoted an unidentified Foreign Ministry official as saying. “The talks will show whether the ties will be cut loose or not, or if a new (Turkish) policy is to be outlined on Syria. That’s the last meeting.”

The Obama administration praised the increased diplomatic pressure. Mark Toner, a State Department spokesman, said Washington was heartened by the response of the Saudis, the Gulf states and the Arab League, but added that more was needed.

There were signs Washington was looking to Turkey to use its influence. Secretary of State Hillary Rodham Clinton called Davutoglu on Sunday and a senior U.S. diplomat, Fred Hoff, visited Ankara, the Turkish capital.

Although the United States and European Union have imposed sanctions on individuals and pressed Assad to reform, they have stopped short of saying he should step down. Western policy makers are concerned about instability in a country with a potent sectarian mix that borders Israel and Lebanon and is allied with Iran.

While Libyan leader Moammar Gadhafi has so far proved able to withstand the end of diplomatic recognition, an armed rebellion and defection of many loyalists, Syria may be different. Unlike Libya, it has little oil revenue to fund its patronage networks and it has in the past proved susceptible to pressure.

“Historically, concerted multilateral pressure and sanctions have the greatest impact on the Assad regime’s calculations,” said Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy and a longtime former resident of Damascus.