Russia and the United States announced a new diplomatic effort Tuesday to bring together the embattled government of President Bashar al-Assad and Syria’s opposition in hopes of ending a conflict now in its third year.

The two nations, which have backed opposing sides in the deepening civil war, said they will push jointly for a new transitional government in Syria. Doing so would represent a new moment of cooperation between the countries, both influential in the Middle East, at a time when the Syrian conflict is severely straining regional stability.

The U.S. and Russian governments have expressed support for an agreement signed last year in Geneva calling for a transition negotiated between the Syrian government and the opposition. Under the deal, each side would have veto power over those chosen to attend.

The Obama administration and its allies, along with the opposition, have interpreted this to mean a guaranteed veto over the participation of Assad and his strongest backers. The Russian government has previously said that would be an unacceptable precondition for the talks.

In announcing the new partnership on Tuesday, however, the top U.S. and Russian diplomats left Assad’s fate vague. The agreement suggested that Russian support for Assad has softened since the emergence of new evidence that his government has probably used chemical weapons on a small scale in the war.

“We are not interested in the fate of certain persons,” Russian Foreign Minister Sergei Lavrov said at a midnight news conference with visiting Secretary of State John F. Kerry. “We are interested in the fate of Syria itself.”

The diplomatic overture here came as President Obama invoked a “moral obligation” for the United States to act in Syria as his administration considers new ways to assist rebel forces seeking Assad’s overthrow.

Obama has supported the rebels and hundreds of thousands of refugees with humanitarian assistance. But he is weighing whether to escalate the U.S. involvement by sending lethal aid, a step that could be avoided if the new U.S.-Russia diplomatic initiative shows progress.

Obama’s considerations gained urgency late last month when the administration acknowledged that there is evidence that chemical weapons are likely to have been used in Syria’s conflict. He has called the use of such weapons a “red line” that would change his approach to the conflict, although in ways he has yet to specify.

Speaking at a news conference Tuesday with visiting South Ko­rean President Park Geun-hye, Obama called for patience, saying he must confirm that Assad used chemical weapons before making any policy decisions based on a “perceived” violation of his red line.

“I don’t make decisions based on ‘perceived,’ ” Obama said, using language from a reporter’s question. “And I can’t organize international coalitions around ‘perceived.’ We’ve tried that in the past, by the way, and it didn’t work out well.”

At the time Obama first called for Assad to step down in August 2011, 2,000 people had died in Syria’s civil war. Today that number exceeds 70,000, and Obama addressed the impatience with his policy at a time when he is seeking to end U.S. involvement in overseas conflicts after more than a decade of war.

“I think that, understandably, there is a desire for easy answers,” Obama said. “That’s not the situation there. And my job is to constantly measure our very real and legitimate humanitarian and national security interests in Syria, but measuring those against my bottom line, which is what’s in the best interest of America’s security.”

Ahead of the meetings in Moscow, U.S. officials acknowledged that Russia has been unwilling to help broker a political resolution that requires Assad’s ouster, as the United States and several European and Arab allies have insisted.

Kerry did not repeat the U.S. demand that Assad leave under any negotiated transition, although he said it is impossible for him to imagine how the Syrian president could remain after accusations of atrocities against civilians.

In the coming weeks, the United States and Russia will convene an international conference that is to include representatives of the Assad government and the diverse opposition.

As part of the U.S. diplomatic effort, the administration is appealing to Russia’s own interests in Syria as the war complicates Russian relationswith Israel and the broader Middle East.

Russia supplies Assad’s government with weapons and has protected it from diplomatic sanctions at the U.N. Security Council. If Russia withdrew that support, Assad would be forced to rely heavily on Iran and the armed Lebanese movement Hezbollah.

Kerry said the administration’s decision on whether to arm Syria’s rebels — a move Obama has resisted — could be avoided if there is progress toward a political deal.

Before the late-night session with Lavrov, Kerry met with Russian President Vladimir Putin, who kept the secretary of state waiting for three hours while he berated his Cabinet for failing to fulfill all the decrees he issued when he regained the presidency a year ago. One of those decrees called on the government to seek closer ties with the United States.

The relationship has worsened, however, with Russia expelling the U.S. Agency for International Development and cracking down on other U.S.-funded activities, and each nation passing tit-for-tat punitive laws.

But Kerry thanked Putin for recent counterterrorism cooperation after the Boston Marathon bombing, highlighting one bright spot. FBI Director Robert S. Mueller III was also in Moscow Tuesday, meeting with Russian security officials.

Wilson reported from Washington. Will Englund in Moscow and Karen DeYoung in Washington contributed to this report.