Syrian government forces are preparing a gradual offensive in and around the northwestern province of Idlib, the last major rebel enclave, says a source close to Damascus.
Russia, the main backing of President Bashar al-Assad from outside the region, said on Wednesday that the militants should be liquidated and Foreign Minister Sergei Lavrov described them as "a purulent abscess."
Turkey, which borders Idlib and has a small military presence there, has warned against such an attack.
The offensive would initially target the southern and western parts of the insurgent territory, but not yet the city of Idlib, said the source, an official of the regional alliance that supports Assad.
"The final touches for the first stage will be completed in the next few hours," the official added, without saying when it would begin.
There are still negotiations on the offensive between Russia and Turkey, as well as with Iran, which also supports Damascus in the war, the official said.
Assad has turned to Idlib, where the jihadist factions dominate, after victories backed by Russia in recent years that drove the rebels from a mosaic of other areas in Syria.
The insurgents now have only the territory in and around Idlib, as well as an adjacent rebel zone in which the opposition authorities are backed by Turkish armor and money, and a piece of desert around a US military camp in the south.
& # 39; Humanitarian catastrophe & # 39;
A major offensive, where displaced people already make up half of the population, risks forcing another 700,000 Syrians out of their homes, says the United Nations.
UN Secretary-General Antonio Guterres warned on Wednesday that large-scale military operations in the Syrian province of Idlib could lead to a "humanitarian catastrophe" and warned against the use of chemical weapons.
"The Secretary-General is deeply concerned about the increasing risks of a humanitarian catastrophe in the case of a large-scale military operation in Idlib province in Syria," said a UN statement.
"The Secretary General reaffirms once again that any use of chemical weapons is totally unacceptable."
He made an urgent appeal to the Syrian government and to all other parties "to act with moderation and give priority to the protection of civilians."
Tensions with Turkey
It also risks increasing tensions with Turkey, whose army established a ring of observation posts throughout the rebel territory last year under a "de-escalation" agreement with Russia and Iran.
Turkish Foreign Minister Mevlut Cavusoglu said earlier this month that he hoped Ankara and Moscow could find a solution for Idlib, warning that a bombing campaign there would be a massacre. Ankara also fears that a major offensive could unleash a new flood of refugees across the Turkish border.
Lavrov said on Wednesday there was a political understanding between Russia and Turkey about the need to distinguish between the Syrian opposition and the people he described as terrorists in Idlib.
According to western estimates, around three million people are at risk in the province, which borders Turkey.
An offensive by the Syrian government, which wants to retake the province with the backing of Moscow, would have explosive repercussions for civilians, as well as for moderate rebels and radical Islamists.
Attack of chemical weapons?
During the closed-door talks at the UN on Tuesday that were requested by Russia, Moscow said, without evidence, that the "white helmet" rescuers were preparing a chemical attack on Idlib, according to diplomats, who called the accusations "very strange". "
Western powers see these claims as an attempt to divert attention from preparations for a Syrian military offensive.
According to diplomats, Western powers have given the UN Security Council the names of Syrian commanders and regiments involved in preparations for an offensive in Idlib, which could be held responsible in case of attacks against civilians.
During a public session on Syria at the UN on Tuesday, Russia denied any possibility of a chemical attack on Idlib by the Syrian government.
The Russian ambassador, Vassily Nevenzia, insisted that the Syrian army does not have chemical weapons and does not intend to use any.