Valery Gerasimov, the head of Russia's general staff, spoke to US counterpart General Mark Milley and the two "exchanged opinions on the situation in Syria and other issues of mutual interest," the Russian defence ministry said in a statement.

No further details were provided.

The 33 Turkish soldiers were killed in a regime air strike in Syria late Thursday, triggering fears of a dangerous escalation.

The attack by Russian-backed Syrian forces took place in the northwestern province of Idlib, where President Bashar al-Assad is waging a campaign to oust rebels from their last holdout.

NATO offered solidarity and support to alliance member Ankara after the strikes but no pledges of concrete new measures to defend Turkey's forces.

Russian and US military officials have periodically discussed the situation in Syria.

After Russia began bombing in Syria in late 2015 to prop up Assad's regime, the US-led coalition and Moscow established a "deconfliction" hotline aimed at preventing the two sides from bombing the others' ground forces or operating in the same air space.

US President Donald Trump last year announced the withdrawal of US troops from northern Syria, where they had been battling the Islamic State group alongside Kurdish forces.

Two days later Turkey launched an offensive into the area.