"We are ready to throw our weight and resources behind diplomacy. We are ready to help bring this conflict to an end," Haley told the UN Security Council.
The US ambassador however added that a US commitment to the peace process was "not enough" and that Washington needed "partners who are serious about using their influence" over Syrian President Bashar al-Assad.
She spoke as US Secretary of State Rex Tillerson was at the Kremlin for talks with Russian President Vladimir Putin following US missile strikes in Syria in retaliation for a suspected chemical attack.
"The United States firmly believes that a political process can work, despite the odds," said Haley.
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The US envoy took aim at Russia, saying it must "stop covering for Assad" and accused Iran of "dumping fuel on the flames of this war in Syria so that it can expand its own reach."
Russia and Iran are providing military support for Assad.
"The road to peace is long. We won't get a political solution overnight. But we can start by working together to actually de-escalate the conflict," she said.
Washington and Moscow "must find a way to work together to stabilise the situation in a deliberate, realistic and concerted way in support of the political process," he said.
"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.
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.
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