Jim Jeffrey told reporters following a closed-door UN Security Council meeting that Moscow and Washington were exploring a "step by step approach" to ending the eight-year war, but that this would require "hard decisions".
During talks in Russia this month, US Secretary of State Mike Pompeo discussed the plan that "would allow a Syrian government that adheres to (UN resolution) 2254 to move back into the international community," said Jeffrey.
Resolution 2254 calls for peace negotiations, the drafting of a new constitution and UN-supervised elections.
But the US envoy said that "so far we haven't seen steps such as a ceasefire in Idlib, the convening of a constitutional committee that would give us confidence that the Assad regime actually understands what it must do to end this conflict."
The United States, which once demanded Assad's ouster, has stopped calling for him to step down, but Jeffrey's remarks suggested it was now ready to offer incentives to help advance prospects for a settlement.
Pompeo held a nearly two-hour meeting with President Vladimir Putin in Sochi on May 14.
Hard decisions
Jeffrey met separately on Wednesday with envoys from fellow permanent Security Council members Britain, France, China and Russia. He said there was a "sincere interest in finding a solution to this conflict."
"But this is going to take hard decisions - hard decisions not only by us, but hard decisions by the Russians and hard decisions most of all by the Syrian regime," he added.
The Security Council met to discuss diplomatic efforts to end the war as Assad's forces and their Russian allies stepped up attacks in the northwestern Idlib region, the last major rebel stronghold.
Russian Deputy Foreign Minister Sergei Vershinin, who is also holding meetings at the United Nations this week, told reporters that Russian-backed Syrian forces were carrying out "targeted operations against terrorists".
Most of Idlib province is controlled by Hayat Tahrir al-Sham, a group dominated by former members of al-Qaeda's Syria affiliate which is on the UN terrorism list.
Vershinin said Russia is "ready to coordinate" with the United States to develop a "common vision about how we want to have a sustainable political settlement in Syria."
UN envoy Geir Pedersen stressed that US-Russia cooperation was key to push ahead with a peace deal for Syria but that the Damascus government must agree to steps.
"Without that, we risk what I call a 'no war, no peace' scenario where things will continue to be complicated and we will not see a Syria that is a normal part of international society in the future," he said.
More than 370,000 people have been killed and half of Syria's population displaced since the war began in March 2011.
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