The Pentagon on Friday urged Russia to keep military channels open after Moscow abruptly hung up a vital hotline that was established to avoid mishaps between the two powers in Syria.

The so-called deconfliction line has been a lifesaving -- albeit imperfect -- tool since it was set up soon after Russia entered Syria's civil war in late 2015 to prop up President Bashar al-Assad.

Even though the US military used the hotline to warn Russia of the impending missile strike on an Assad air base near Homs early Friday, a furious Moscow reacted to the attack on its ally by saying it would no longer cooperate with the Americans.

"The Russian side is halting the effect of the memorandum for prevention of incidents and ensuring safety of air flights during operations in Syria which was agreed with the US," the Russian foreign ministry said.

Pentagon spokesman Major Adrian Rankine-Galloway said he hoped Russia would reconsider.

"The Department of Defense maintains the desire for dialogue through the flight safety channel," he said.

"It is to the benefit of all parties operating in the air over Syria to avoid accidents and miscalculation, and we hope the Russian Ministry of Defense comes to this conclusion as well."

The hotline was established between US officers monitoring the war from an operations center at a base in Qatar, and their Russian counterparts operating in Syria.

The link is just a regular phone line staffed on the US side by a Russian-speaking colonel and has been used almost daily since its inception.

Moscow's move to abandon the hotline could dramatically raise the risk to pilots and ground forces on all sides.

It was used in February to stop a Russian strike on US-backed fighters in several small villages in northern Syria after the Russian pilots apparently mistakenly thought Islamic State (IS) militant group forces were in the area.

Further heightening tensions, Russia's military said Syrian air defenses would be strengthened to shield the country's key infrastructure.