“Iran, in a lot of ways, has no Plan B,” said Alireza Nader, an expert on Iran at the Washington offices of the RAND Corporation, a research group. “I think the main issue is that Iranian foreign policy has become so rigid that it’s hard to come up with a Plan B. They’ve built this framework of resistance that’s hard to escape.”

Mr. Jalili’s visit, which was also an attempt to secure the release of 45 Iranian hostages held by Syrian rebels, appeared to strain relations with Turkey, an important trading partner that has been alienated by Mr. Assad. The Foreign Ministry in Turkey — where Iran’s foreign minister, Ali Akbar Salehi, was visiting — issued a strong rebuke.

“It is unacceptable and irresponsible that Iranian officials in various posts continue to target our country through their statements,” the Foreign Ministry said in a statement. Seeking to shift blame back to Mr. Assad, it added: “Everyone knows who, inside and outside Syria, is responsible for the human tragedy caused by the Syrian regime. They will be called to account by history and human conscience.”

Secretary of State Hillary Rodham Clinton, traveling in South Africa, also warned against turning Syria into a proving ground for outside interests. “Those who are attempting to exploit the misery of the Syrian people, either by sending in proxies or sending in terrorist fighters, must recognize that that will not be tolerated, first and foremost by the Syrian people,” she said.

Accounts of the Assad-Jalili meeting in Syria’s state news media emphasized the view that Mr. Assad’s government was resilient and in control. Syria television reported that Mr. Assad had assured Mr. Jalili that the Syrian people would make sure that “foreign projects” in Syria failed as the government pursued its enemies “without complacency.”

According to Iranian news reports, Mr. Jalili told Mr. Assad that “we should not allow the enemies to take revenge” for what Iran views as the defeat of Israel in 2006 and 2009 in wars with “the resistance front,” a strategic alliance of the Iran, Syria, the Palestinian Islamic Jihad Movement and the Hezbollah organization in Lebanon.