Russian Foreign Minister Sergei Lavrov on Wednesday described militants in Syria's last big rebel-held enclave of Idlib as a "festering abscess" that needed to be liquidated.

Speaking after talks with his Saudi counterpart Adel bin Ahmed al-Jubeir in Moscow, Lavrov said militants were using civilians as a human shield.

Lavrov told reporters that there was a political understanding between Turkey and Russia on the need to distinguish between the Syrian opposition and people he described as terrorists in Idlib Province.

The government of Syrian President Bashar al-Assad, backed by Moscow, says it aims to recapture Idlib, which has become a refuge for civilians and rebels displaced from other areas of Syria, as well as jihadist forces.

The region has been hit by a wave of air strikes and shelling this month in a possible prelude to a full-scale government offensive.

Russia has deployed several frigates to the Mediterranean via the Bosphorus, part of what a Russian newspaper on Tuesday called Moscow's largest naval buildup since it entered the Syrian conflict in 2015.

Lavrov said that Moscow was in close contact with Turkey on the situation in Idlib.

"This is the last hotbed of terrorists who are trying to speculate on the region's status as a de-escalation zone, who are trying to hold the civilian population hostage as human shields and bend to their will those armed groups ready to engage in dialogue with the government," Lavrov said.

"So from all points of view, this festering abscess needs to be liquidated," he said.

Lavrov also said Russia remained in contact with the United States on the situation in Idlib and that communication was happening between their two militaries.