The United Nations peace envoy for Syria on Wednesday urged the United States and Russia to agree on a way forward to end the war in Syria and pave the way for a "real negotiation."

Special envoy Staffan de Mistura told the UN Security Council that he was ready to convene a new round of talks in May but that US-Russian cooperation was needed.

His appeal came as US Secretary of State Rex Tillerson was in talks in Moscow with his Russian counterpart Sergei Lavrov following US missile strikes in Syria in retaliation for a suspected chemical attack.

The two countries have "serious differences" but also a "common interest" in ending the six-year war that has killed 320,000 people, De Mistura said.

"They must find a way to work together to stabilize the situation in a deliberate, realistic and concerted way in support of the political process," he said.

The United Nations is ready to lead a "real negotiation" on a "package for an orderly, mapped-out, credible and irreversible, agreed political transition" to end the war, he added.

The council was meeting ahead of a vote later in the day on a draft resolution demanding that the Syrian government cooperate with an investigation of the suspected chemical attack in the rebel-held town of Khan Sheikhun.

Russia is expected to veto the measure -- which would mark the eighth time that Moscow has used its veto power to block Security Council action directed at its ally, Syria.

"Let us view this moment of crisis -- and it is a moment of crisis -- as a watershed and an opportunity for a new level of seriousness in the search for a political solution," said De Mistura.