REUTERS - President Barack Obama said the United States and Russia were working on Sunday to try to finalize a ceasefire in Syria that would allow more deliveries of humanitarian aid in the war-torn country.

"We're not there yet," Obama told reporters after a meeting with UK Prime Minister Theresa May on the sidelines of the G20 summit in the eastern Chinese city of Hangzhou, noting previous ceasefires had failed to last for long.

Military officials from the United States and Russia, which back opposite sides in Syria's five-year war, have been meeting for weeks to try to work on terms of a deal.

Open gallery view Russian President Vladimir Putin meets with Chinese President Xi Jinping (not in picture) ahead of G20 Summit in Hangzhou, Zhejiang province, China, September 4, 2016. Credit: Wang Zhau, Reuters

The civil war has killed more than 250,000 people and displaced 11 million, causing a refugee crisis in the Middle East and Europe, and contributing to a rise in militant Islamist groups.

"We have grave differences with the Russians in terms of both the parties we support but also the process that is required to bring about peace in Syria," Obama said.

Russia has backed Syrian President Bashar Assad, but the United States has worked with moderate opposition forces fighting Assad.

"But if we do not get some buy-in from the Russians on reducing the violence and easing the humanitarian crisis, then it's difficult to see how we get to the next phase," he said.

The White House has said Obama and Russian President Vladimir Putin likely would have the chance to talk informally on the sidelines of the G20. No time for that meeting has been announced.

Speaking later to reporters after meeting Russian Foreign Minister Sergei Lavrov, U.S. Secretary of State John Kerry said a couple of tough issues still remained and that the two sides would meet again on Monday to try to bridge the gap.

"It's fair to say that out of the review I think there are a couple of tough issues that we talked about today," Kerry said, declining to give details.

"We will meet tomorrow morning and see whether or not it is possible to bridge the gap and come to a conclusion on these couple of issues."

Reporters had gathered in a small conference room with two lecterns set up side by side, awaiting a potential deal.

Moments before Kerry entered, aides hurriedly removed one of the lecterns, making it clear a deal would not be in the offing.

Post-Brexit trade talks

In his first meeting with May since she took office in Britain, Obama sought to demonstrate American solidarity with the U.K. amid the tumult over its decision to leave the European Union. He and May both said their countries would continue an ambitious trade agenda together, though Obama conceded that Britain's first task was to figure out its new trade relationship with its EU neighbors.

Obama's previous suggestions that the U.S. would prioritize ongoing U.S.-EU trade talks over a one-on-one deal with Britain rankled London, but Obama said it was never intended to be a punishment.

"The bottom line is we don't have a stronger partner in the world than the United Kingdom," Obama said. "And despite the turbulence of the political events over the last several months, we have every intention of making sure that continues."

May, echoing Obama's commitment to continue close cooperation with the U.S. on economic and security issues, sought to put to bed any notion that the U.K. would hold another referendum or reverse course on the EU exit, or Brexit.

"The U.K. will indeed be leaving the European Union," she said.