With the United States and its allies pressing President Bashar al-Assad to step down, the Arab League last week issued a detailed plan for a political transition in Syria. The plan was welcomed by the Obama administration, and Arab leaders quickly said they would refer it to the United Nations.

And a day later, Russia had its say: Not a chance.

“This is an effort from the Arab League, if I understand correctly, to sort of already put a precooked solution on the table,” said Vitaly I. Churkin, Moscow’s envoy to the United Nations. “I understand that the attitude of Damascus to that has been negative.”

The response doomed any hope of a quick resolution at the United Nations to bring greater pressure to bear on the Syrian government, but it also fell into a familiar pattern by which Moscow has shown a growing willingness to challenge the United States and its European partners on a range of issues.

In recent weeks, Moscow has sought U.N. scrutiny of possible crimes by NATO during its air campaign in Libya, and even called for investigations into organ sales in Kosovo, a close ally of the West. Most notably, Moscow has obstructed any effort to increase pressure on Iran.

Asked in an interview whether Russia would ever support a U.N. resolution imposing economic sanctions on the Islamic republic, Churkin said: “No chance, no chance, no chance. . . . Ever.”

Critics say Moscow’s tough line at the United Nations reflects what one senior council diplomat described as “the Putinization of Russian foreign policy,” on the eve of what many expect will be the return of Vladimir Putin to the presidency. Other analysts say Russia is trying to reassert its authority in the council following a period in which the United States and Europe prevailed in the handling of several major crises, engineering the downfall of former Ivory Coast leader Laurent Gbagbo and, more recently, of Moammar Gaddafi in Libya.

“The Russians looked diminished in the first half of 2011, and the strategy is to show, one, they are prepared to act as a spoiler, but, two, they can also lay out a more proactive agenda,” said Richard Gowan, an expert on the United Nations at the New York University Center for International Cooperation.

The United States and its European partners have responded to Russian aims by mounting a campaign to isolate it at the United Nations, and portraying Moscow as an obstacle to the democratic changes sweeping through the Middle East.

This past week, for example, the United States, Britain, Germany and France publicly rebuked Syria’s arms suppliers, a veiled reference to Russia, for continuing to sell weapons to Damascus.

“It is glaringly obvious that transferring weapons into a volatile and violent situation is irresponsible and will only fuel the bloodshed,” Britain’s U.N. envoy, Mark Lyall Grant, told the Security Council.

Russia is coming under mounting pressure to break with Assad from the Arab League, which is sending a delegation to the Security Council on Tuesday to press its case for a political transition that would require that the Syrian leader step aside. Meanwhile, Morocco, acting on behalf of a group of Western and Arab governments, has introduced a draft resolution endorsing the Arab League initiative calling on states to follow the Arab organization’s example by imposing sanctions on Syria. The Russians have responded coolly.

The high-level diplomatic gamesmanship is playing out as violence continues to spiral in Syria, forcing the United States to prepare for the possible closure of its embassy and the evacuation of its diplomatic personnel. U.N. officials estimate that more than 5,400 civilians have been killed, mostly at the hands of government security forces, since protesters took to the streets earlier last year.

Russia’s stance underscores the strength and depth of its relationship with Assad’s regime, which is not only a recipient of Russian arms but also host of a Russian naval base. The crisis in Syria also has provided Moscow with an opportunity to show it is a more reliable ally than Western powers, particularly the United States, which is seen by many in the region as having abandoned one of its closest allies, former Egyptian leader Hosni Mubarak.

“If you have good relations with a country, a government for years, for decades, then it’s not so easy to ditch those politicians and those governments because of political expediency,” Churkin said. “We are stronger on our allegiances than others.”

Tensions between the West and Russia have spilled over into a series of highly personal attacks at the United Nations. Last month, U.S. Ambassador Susan E. Rice derided Churkin’s appeal for a new probe into possible NATO killings as a “cheap stunt” aimed at distracting attention from the killing in Syria.

Churkin fired back at the Stanford-educated envoy, saying, “Really this Stanford dictionary of expletives must be replaced by something more Victorian.” Rice’s media spokesman posted a tweet with a photoshopped picture of Churkin on the head of “the Grinch Who Stole Christmas.”

Asked if he was trying to change the subject from Syria, Churkin acknowledged that the Security Council has been the scene of “games of distraction,” but he said Russia’s concerns about the Libya mission were legitimate, asserting that the killing of civilians during the NATO campaign was “a real issue.”

Russia, along with Brazil, China, India and South Africa, believe “it would be extremely dangerous if” the West continues to be “carried away by this regime-change idea,” he said.

In Syria, Russia has pursued a complicated diplomatic strategy to shore up the regime, joining China in vetoing the Western-backed resolution threatening sanctions against Damascus, and introducing its own resolution. That resolution, now stalled, sought to focus the Security Council’s energy on backing a political settlement between the Syrian government and the opposition, and cutting off military supplies to the opposition.

Under the resolution, Assad’s army could still be armed.

Asked if Russia’s ongoing arms sales to Assad’s government were perhaps undercutting his government’s effort to pursue a political settlement, Churkin said: “We are not doing anything which is contrary to international law. Other than that, we don’t have to give any explanation to anybody.”