The United States and Russia are nearing an agreement on Syria for how they hope to resolve the Arab country's civil war once the Islamic State group is defeated, officials said on Thursday.

If clinched, the deal was expected to be announced by President Donald Trump and Russian President Vladimir Putin in Vietnam on Friday, four US officials said.

The United States has been reluctant to schedule a formal meeting for the leaders unless they have a substantive agreement to announce.

But White House press secretary Sarah Huckabee Sanders said on Friday that they will not hold a formal meeting due to scheduling conflicts on "both sides."

Still, Ms Sanders said it was possible Mr Trump and Mr Putin could have a less formal encounter while in Vietnam.

The potential understanding comes as an array of forces are near a final defeat of IS, 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 Assad's government and rebels - and to concerns that foreign powers such as Iran will now dominate the country's future.

The US-Russian agreement being discussed would focus on three elements, officials said: "de-confliction" between the US and Russian armed forces, 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 armed forces have maintained a "de-confliction" 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 Mr Assad, a close ally of Moscow.

With IS 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 M Trump held his first meeting with Mr Putin in Germany, the US and Russia announced a deal that included Jordan and established a cease-fire in southwest Syria.

The United States has said that cease-fire has largely held and could be replicated elsewhere in the country.

Sign up to our free newsletters

Get the best updates straight to your inbox:
Please select at least one mailing list.

You can unsubscribe at any time by clicking the link in the footer of our emails. We use Mailchimp as our marketing platform. By subscribing, you acknowledge that your information will be transferred to Mailchimp for processing.