Russia has deployed at least a dozen vessels to waters off Syria ahead of the final regime offensive against rebels, in what is believed to be the largest Russian naval buildup in the Mediterranean since the nation entered the conflict in 2015.

Russian media reported at least two submarines and 10 warships were dispatched to the area this week. Lebanese daily Al-Masdar – which supports Syrian president Bashar al-Assad – said there were as many as 17 Russian vessels off the Syrian coast.

Frigates Admiral Grigorovich and Admiral Essen, which are armed with Kalibr long-range cruise missiles, were photographed on Saturday moving through the Turkish straits connecting the Black Sea and the Mediterranean.

A day earlier, the Pytlivy frigate and landing ship Nikolai Filchenkov were also pictured taking the same route. The Vishny Volochek missile corvette passed through the Bosphorus this month.

The massive deployment came as President Assad’s forces, backed by Russian land, sea and air power, prepared to take Idlib, the rebels’ last major enclave in Syria.

The anticipated battle, in the north of the country, is expected to be the final showdown in the seven year conflict. Regime forces have successfully swept back into control of most the country since Russia intervened in 2015, turning the tide in President Assad’s favour.

The United Nations warned on Tuesday a large scale offensive in Idlib could spark a humanitarian emergency “at a scale not yet seen” in the conflict.

The northwestern province, near the border with Turkey, is home to almost 3 million people. It has a considerable al-Qaeda and jihadist presence but is also a refuge for thousands of civilians and rebels displaced from other areas of Syria.

John Ging, the UN’s director of humanitarian operations, warned recent weeks had seen a significant deterioration in the humanitarian situation, amid intense bombing and shelling in parts of Idlib as well as Aleppo, Hama and Latakia provinces, resulting in civilian deaths and the destruction of schools and hospitals.

Many speculate the massive Russian naval presence near Syria is not only in preparation for the anticipated battle, but intended as a deterrent to the United States, the UK and its allies, if Assad’s troops use too much force, or even chemical weapons.

On Wednesday, Russia delivered a veiled warning to the West not to stand in the way of an “anti-terror operation” in Idlib.

At a press conference in Moscow with Saudi officials, Russian foreign minister Sergei Lavrov called fighters in Idlib a “festering abscess”. Russia, Assad and its Iranian and Lebanese allies dismiss the rebel forces in Idlib as jihadis and extremists.

“I hope our Western partners will not give in to [rebel] provocations and will not obstruct an anti-terror operation... This abscess needs to be liquidated,” Mr Lavrov said.

He added he was in contact with the US about the situation.

I hope our Western partners will not give in to [rebel] provocations and will not obstruct an anti-terror operation... This abscess needs to be liquidated Sergei Lavrov, Russian foreign minister

A day earlier, US State Department officials and the UK warned Russia and Syria they would retaliate if chemical weapons were used on Idlib, or elsewhere in the country.

State Department spokeswoman Heather Nauert said US officials made it “very clear to Damascus” a response would be delivered “in a swift and appropriate manner”.

US secretary of state Mike Pompeo told Mr Lavrov last week Moscow would be held responsible, if chemical weapons were deployed.