President Barack Obama said on Monday the US and Russia have not given up on negotiations that could stem the bloodshed in Syria, but acknowledged that leaders are challenged by "gaps of trust" between the rival powers.

Significant sticking points remain in the negotiations over creation of an unlikely US-Russian military partnership focusing firepower on "common enemies" in Syria, Obama said.

He acknowledged that a flurry of diplomacy at an economic summit and a 90-minute meeting earlier on Monday with his Russian counterpart, Vladimir Putin, did not yield a breakthrough.

"Given the gaps of trust that exist, that’s a tough negotiation," Obama said at a news conference closing the Group of 20 summit. "We haven’t yet closed the gaps." 

Obama didn’t detail the trouble spots, although he suggested the US has concerns about Russia holding up its end of the bargain and enforcing the terms.

Any deal would depend on Moscow using its influence with Syrian President Bashar Assad to persuade him to ground planes and stop the assault on opposition forces.

Obama said the aim was to reach "meaningful, serious, verifiable cessations of hostilities in Syria." 

Secretary of State John Kerry and Russian foreign minister Sergey Lavrov have for weeks been trying to broker a deal that would curb the violence between the Russian-ally Assad’s government forces and moderate rebels backed by the US Talks are expected to resume quickly, probably later this week.

The deal depends on the two sides agreeing to closer military coordination against extremist groups operating in Syria, something the Russians have long sought and the US resisted.

Obama has expressed skepticism that Russia would hold to its agreement. The State Department has said it wants a nationwide cease-fire between Assad’s military and the rebels, rather than another time-limited agreement like ones that failed before.

Obama’s meeting with Putin came as the Russian leader is playing a prominent role in the presidential campaign at home.

US officials blame Russian intelligence for a hack on the Democratic National Committee that resulted in a leak of emails damaging to its presidential nominee, Hillary Clinton.

Putin has denied his government was involved, but cheered the release of the information.

The president expressed concerns to Putin about cybersecurity issues, but would not detail the discussions.

Obama and Putin also discussed the conflict in Ukraine, where Russian-backed separatists have been fighting the government, and the implementation of an agreement to stop the violence.

Obama met earlier with French President Francois Hollande and German Chancellor Angela Merkel on the same issue.

The difficult diplomacy on Syria set the tone for an uneven few days for Obama on his final tour of Asia as president.