The US and Russia have signed an agreement to minimise the risk of incidents in Syrian airspace, says the Pentagon.

The memo of understanding includes protocols for air crews in Syrian skies along with a ground communications link, according to US officials.

Tuesday's pact doesn't include zones of co-operation or sharing of target information, said Department of Defense spokesman Peter Cook.

Moscow insisted the text of the agreement remain secret, he added.

Russian defence ministry spokesman Maj Gen Igor Konashenkov confirmed they had received the US memo, describing it as an important step forward.

The US has been pursuing a technical agreement with the Russians on how both nations' air forces can keep a safe distance from each other.

On two recent occasions over Syria, US officials have said Russian pilots flew within a few hundred feet of American warplanes. 

Moscow have said one of its Su-30 jets approached a US warplane over Aleppo on 10 October, not to scare it off but to visually identify the bogie.

Syria's skies have become dangerously crowded of late as the US, Russia and Syria's regime all bomb various factions in the country's four-year-old civil war.

A US-led international coalition has been waging an intensive air campaign since autumn last year against the Islamic State group and al Qaeda in Syria.

Russian President Vladimir Putin last month dispatched warplanes to the region to target what he said were Islamic State militants.

But US defence officials say most of Moscow's air strikes have been targeting secular rebels in a bid to aid Kremlin ally President Bashar al Assad.

Russia's air force in Syria is composed of Sukhoi warplanes - including the Su-30, Su-25 and Su-24 - which aviation analysts rate as some of the best in the world.

The US has been primarily using F-22 Raptor jets and F-16s launched from Turkey's Incirlik air base.