The United States and Russia are nearing an agreement on how they hope to resolve Syria's civil war once Isis (Islamic State) is defeated, officials said yesterday.

If clinched, the deal could be announced by US President Donald Trump and Russian President Vladimir Putin in Vietnam this weekend, four US officials said.

The potential understanding comes as an array of forces are near a final defeat of Isis, the extremist group that once controlled vast stretches of both Iraq and Syria.

Fighting the group is no longer top priority, shifting the focus back to Syria's intractable conflict between President Bashar al-Assad's Government and rebels - and to concerns that foreign powers such as Iran will now dominate the country's future.

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The US-Russian agreement being discussed would focus on three elements, officials said: "deconfliction" between the US and Russian militaries, reducing violence in the civil war and reinvigorating UN-led peace talks.

The officials weren't authorised to discuss the deliberations and requested anonymity.

The US and Russian militaries have maintained a "deconfliction" hotline for years to avoid unintended collisions and even potential confrontations as they each operate in Syria's crowded skies.

A heavy air campaign by Russia has been credited with shoring up the position of Assad, a close ally of Moscow.

With Isis nearing defeat, the US and Russia are losing their common enemy in Syria and will remain in a proxy battle in which Russia backs Assad and the US lends at least rhetorical support to armed opposition groups fighting the Government.

That has increased the need for close communication between the two powers about where their forces are operating at any given time, officials said.

The agreement also seeks to build on progress in establishing "de-escalation zones" in Syria that have calmed some parts of the country.

In July, when Trump held his first meeting with Putin in Germany, the US and Russia announced a deal that included Jordan and established a ceasefire in southwest Syria.

The US has said that ceasefire has largely held and could be replicated elsewhere in the country.

A key US concern, shared by close ally Israel, is the presence of Iranian-backed militias in Syria that have exploited the vacuum of power.

The US and Israel have been seeking ways to prevent forces loyal to Iran - Israel's archenemy - from establishing a permanent presence. One idea hinges on a "buffer zone" along Israel's border with Syria.

A third element of the deal would reaffirm support for the United Nations effort being run out of Geneva to seek a political transition in Syria and resolve the civil war.

The US and Russia have been at odds for years over whether Assad could be allowed to remain in power in a future Syrian government.

The UN talks, which have come in fits and starts without yielding significant progress, aren't the only discussions about Syria's future.

Russia, Turkey and Iran have been brokering their own process in Astana, Kazakhstan.