Moscow and Washington support opposite sides in the Syrian conflict, which erupted in March 2011.
Successive rounds of international negotiations have failed to end a five-year conflict that has left more than 290,000 people dead and forced millions to flee, a key contributor to migrant flows into Europe.
“It is a very complicated piece of business,” Obama told reporters.
Russia is one of President Assad’s most important international backers while the US supports Syria’s main opposition alliance and some rebels, with other countries and forces also involved.
“Trying to corral all of those different forces into a coherent structure for negotiations is difficult,” Obama said after meeting British premier Theresa May ahead of the G20 summit.
“But our conversations with the Russians are key.”
A senior State Department official said a deal was close, and could be announced by Secretary of State John Kerry and his Russian counterpart as soon as Sunday, but some issues remained to be resolved.
“John Kerry and his counterpart, Sergei Lavrov, have been working around the clock, as well as a number of other negotiators to see what would a real cessation of hostilities would look like,” Obama said.
“We’re not there yet,” AFP reported.
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