The US is sceptical an agreement with Russia to end violence in Syria can work but will keep pursuing it nonetheless, president Barack Obama said as negotiators from both countries edged towards a deal.
Mr Obama, speaking on the sidelines of the G20 summit in China, said the US and Russia still have "grave differences" about what is needed to end Syria's civil war and which opposition groups are legitimate targets for the US and Russian forces. But he said "it is worth trying".
"We're not there yet," Mr Obama said. "I think it's premature for us to say there's a clear path forward, but there's the possibility at least for us to make some progress."
A deal could be announced as early as Sunday by US secretary of state John Kerry and Russian foreign minister Sergey Lavrov, said a senior US state department official, adding that the two countries were close to a deal but still had unresolved issues.
Mr Kerry and Mr Lavrov have been deep in talks for weeks over a deal to boost US and Russian military cooperation to fight the Islamic State group and other extremists in Syria - a step Moscow has long sought.
The package would include provisions so aid can reach besieged areas of Syria and measures to prevent Syrian president Bashar Assad's government from bombing areas where US-backed rebels are operating.
US officials have said that as part of a deal, Russia would have to halt offensives by Assad's government, something it has failed to do over months of diplomatic efforts.
They said the US must get rebels to break ranks with the al Qaida-linked Nusra Front, a task that grew tougher after Nusra fighters last month successfully broke the siege of Aleppo, Syria's largest city and the site of recent fierce fighting.
Though negotiators have been hopeful a deal could come together while world leaders are gathered in Hangzhou for the G20, that optimism has been tempered by the failure of previous ceasefire deals to hold.
The US has long been wary of increasing military coordination with Russia in Syria's civil war because it says Russia continues striking moderate, US-backed opposition groups in a bid to prop up Assad. The US wants Russia to focus exclusively on IS and al Qaida-linked groups.
Mr Obama and Russian president Vladimir Putin plan to meet on the sidelines of the summit, the White House said.
For Mr Obama, a military partnership with Russia would mark a significant change. When Russia started bombing targets in Syria last year, the US declared the intervention an act of desperation and said its coalition fighting IS was not coordinating with Moscow. The minimal cooperation focused on avoiding mid-air collisions between Russian and coalition planes.
The new approach would involve intelligence and targeting cooperation. Assad's forces would be barred from attacking areas outside of IS control; attacks on Nusra and its allies would be up to the US and Russia to work out among themselves in their Joint Implementation Group. Defence secretary Ash Carter and National Intelligence director James Clapper both have expressed misgivings.
Discussions about the intractable Syria conflict and the related fight against IS have been a major focus as world leaders gather for the G20, which brings together the world's major economies.
Mr Obama met first with new Prime Minister Theresa May, then with Turkish president Recep Tayyip Erdogan for their first sit-down since the failed coup in his country in July.
Mr Obama called the attempted overthrow "terrible." He assured Mr Erdogan that his national security team and the justice department would ensure that those responsible are brought to justice, a reference to Turkey's extradition request for an exiled cleric it holds responsible. The US is still weighing Turkey's evidence against Pennsylvania-based cleric Fethullah Gulen.
Turkey's demands for the US to hand over Mr Gulen have coincided with growing clashes between Turkish forces and US-backed Kurds in Syria. The Pentagon has backed the incursions, but said they should only be aimed at IS fighters. Turkey has used the operations to push back Syrian Kurds it accuses of seeking to claim more territory.
Mr Obama called Turkey a key ally in the campaign to defeat Islamic State and said "we now need to finish the job" of securing Turkey's border with Syria.
Since the failed coup, the US has been alarmed by Turkey's diplomatic flirtations with Russia, Assad's patron, and softened demand for Assad's exclusion from a political transition.
Mr Erdogan said it was important for the US and Turkey to "embrace a common attitude against terrorism". In a reference to Washington's support for the Kurds, he said there are "no good terrorists or bad; all terrorism is bad".