"This line exists and it's in an active state," Kremlin spokesman Dmitry Peskov told journalists, adding that the line intended to prevent accidental clashes and midair near misses "is used by both sides."

The hotline's suspension has been announced by Russia several times during periods of tension between the countries carrying out separate bombing campaigns.

The US in March said it had been used to narrowly avert a clash when Russian soldiers came too close to positions of US soldiers, without giving a precise date for the incident.

US Defence Secretary Jim Mattis said that the chairman of the US Joint Chiefs of Staff, General Joe Dunford, contacted his Russian counterpart, General Valery Gerasimov, about the incursion.

The line, set up by a memorandum signed with the US in 2015, is also known as a "deconfliction line" because it is used to maintain a zone of "deconfliction" between the two sides.

The US used the hotline in April last year when it fired its first missile strike at a Syrian airbase.

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

The link is a regular phone line staffed on the US side by a Russian-speaking colonel.

The hotline has become ever more vital as Assad's forces have gained ground over rebels in Syria.