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US Secretary of State John Kerry will make his latest trip to Moscow this week to seek common ground on how to deal with the ongoing bloodshed in Syria.

The State Department said Kerry would speak to his Russian counterpart about the wars in Syria and Ukraine.

Spokesman John Kirby played down reports that Washington and Moscow may agree on coordinated US-Russian military action against militant groups al-Nusra Front and the Islamic State group [IS] in Syria.

"But, as we've said before, we continue to explore options, alternatives and proposals with respect to the fight against Nusra and [IS] in Syria," he said.

"And... to the degree to which the Russian military is willing to be committed to the fight against those two groups, and exclusively those two groups, that's a conversation that we're willing to have," Kirby added.

"But there's no military coordination going on now."

There was no word as to who Kerry will meet in Moscow on Thursday and Friday, but on previous such trips, he has met with Foreign Minister Sergei Lavrov before heading to the Kremlin to see President Vladimir Putin.

Russian forces are in Syria to support President Bashar al-Assad's regime in its five-year-old civil war against a variety of rebel forces, whereas a US-led coalition is focused on fighting IS.

Washington has repeatedly urged Moscow to pressure its ally into agreeing on a ceasefire with so-called "moderate" militants and to enter peace talks to end the civil war, but fierce fighting has continued.

The UN envoy for Syria said on Monday a "crucial moment" had been reached in attempts to broker a political settlement to end the war and defeat IS.

"It is a crucial moment," Staffan de Mistura said, adding that "the key lies in a possible agreement between Russia and the United States".

"They have proved themselves to be able to agree and the others will adapt," he said, referring to previous successes such as a truce in February between regime and non-jihadist rebel groups which was brokered by Russia and the US.

Agencies contributed to this report.