US secretary of state John Kerry met with Russian President Vladimir Putin about boosting military and intelligence cooperation against Islamic State and al-Qaeda in Syria and told him that without "concrete, near-term steps", diplomatic efforts to end the war could not go on indefinitely.
Kerry met Putin for three hours at the Kremlin and their talks lasted until 1am local time on Friday.
The state department said Kerry expressed concern about repeated violations of a cessation of hostilities by the Moscow-backed Syrian government. It said the two also discussed the need to increase pressure on groups like Islamic State and the Al-Nusra Front, al-Qaeda's affiliate in Syria.
Kerry "emphasized that absent concrete, near-term steps, diplomatic efforts could not continue indefinitely", a statement said, adding that discussions between Kerry and Russian foreign minister Sergei Lavrov on Friday were expected to explore initiatives in more detail.
On Thursday, the Washington Post published a leaked document it said Kerry would put forward in Moscow calling for intelligence sharing to identify leadership targets, training camps, supply lines and headquarters of the Nusra Front, al Qaeda's affiliate in Syria.
It said strikes against those targets could be carried out by US or Russian jets and expanded coordination would be channelled through a Joint Implementation Group based in the vicinity of the Jordanian capital, Amman.
The extent of cooperation proposed in the document would represent a major US shift after years of rivalry between Washington and Moscow, which support opposing sides in Syria's five-year civil war, but the idea has raised doubts among US defense and intelligence officials.
Putin said at the start of his meeting with Kerry that his last conversation with US President Barack Obama had convinced him that both sides were sincere in their efforts to find a solution in Syria.
"I hope after today's consultations you'll be able to advise him of the progress made and possible headway for us to make," he told Kerry.
A test of Russia
US officials described the visit as a test of Moscow's willingness to use its influence on the Syrian government to help revive the country's peace process.
Under the leaked plan, the US and Russia would establish separate headquarters and a shared coordination office, where they would deploy senior officials, intelligence personnel and experts in strike planning and targeting.
They would decide on a date to simultaneously begin strikes against Al-Nusra Front targets and to stop all Syrian military air activities in designated areas, except for non-combat purposes and against areas where Nusra Front has acquired territory.
The proposal also allows for Russia to use air power to defend Syrian forces from attack from AL-Nusra Front within a designated area, if agreed in advance with the United States.
The White House and Pentagon offered lukewarm support for the plan and demanded that Moscow show it was serious about fighting Islamic State, not just propping up President Bashar al-Assad. Kerry himself took a tough line after his meeting on Thursday with Putin, state department spokesman John Kirby told reporters in Moscow.
"Secretary Kerry emphasized that absent concrete, near-term steps, diplomatic efforts could not continue indefinitely," Kirby said.
A senior US official said before the talks that expectations were "very low", but added: "Either we find a way to do something about it or not.
"And if we don’t, the entire thing breaks down. That would be an end of the cessation of hostilities and that would not be a good thing for Russia, or the United States, or the world, or, most importantly, for the Syrian people."
Kerry faces some strong opposition to his efforts to woo Russia from US defense and intelligence officials who say Washington and Moscow have diametrically opposed objectives in Syria.