Syrian government forces are preparing a phased offensive in the northwestern province of Idlib and surrounding areas, the last big rebel enclave, a source close to Damascus said on Wednesday.
Russia, President Bashar al-Assad's main backer from outside the region, said on Wednesday that militants there must be liquidated and Foreign Minister Sergei Lavrov described them as "a festering abscess".
The offensive would initially target southern and western parts of the insurgent territory, but not yet Idlib city, said the source, an official in the regional alliance backing Assad.
"The final touches for the first stage will be completed in the coming hours," the official added, without saying when it would start.
There were still negotiations over the offensive going on between Russia and Turkey, as well as with Iran which also supports Damascus in the war, the official said.
Turkey, which borders Idlib and has a small military presence there, has warned against such an attack.
At the U.S. State Department, spokeswoman Heather Nauert said there were indications that Syrian forces were preparing an offensive on Idlib.
She said Washington's new representative for Syria, Jim Jeffrey, raised concerns about the possible offensive in a recent meeting with Russian ambassador to the United States.
"We are concerned not just about a potential chemical weapons attack, but we are concerned about any kind of escalation of violence in Idlib that would put civilians and civilian infrastructure in Idlib at risk," she added.
"We have shared the concerns we have about any potential offensive taking place with the Russian government at many levels," Nauert told reporters.
Assad has turned towards Idlib, where jihadist factions dominate, after Russian-backed victories in recent years that drove the rebels from a mosaic of other areas in Syria.
The insurgents now hold only the territory in and around Idlib, as well as an adjacent rebel area in which opposition authorities are backed by Turkish armour and money, and a patch of desert around a U.S. military camp in the south.
A major offensive in the Idlib area, where displaced people already make up half the population, risks forcing another 700,000 Syrians from their homes, the United Nations said.
It also risks raising tensions with Turkey, whose army established a ring of observation posts around the rebel territory last year under a "de-escalation" deal with Russia and Iran.
Turkish Foreign Minister Mevlut Cavusoglu said this month he hoped Ankara and Moscow could find a solution for Idlib, warning that a bombing campaign there would be a massacre. Ankara also fears 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 on the need to distinguish between the Syrian opposition and people he described as terrorists in Idlib.
The first phase of the offensive will include the town of Jisr al-Shughour and the al-Ghab plain on the western side of the rebel territory, and the towns of al-Latamenah, Khan Sheikhoun and Maarat al-Numan in its south, the official said.
Taking those areas would bring Assad close to regaining control over highways running from Aleppo to Hama and Latakia, two of Syria's most important pre-war roads.
The army and its allies would also reinforce the western edge of Aleppo city in anticipation of a rebel assault there aimed at blunting the offensive, the official said.
In recent weeks, the Syrian military has dropped leaflets over Idlib province urging people to surrender, opened a route for civilians to cross into government territory and, according to a war monitor, sent more reinforcements to the front. It has stepped up air strikes and shelling near the front lines, the Syrian Observatory for Human Rights has said.
Idlib and surrounding areas outside government control are home to 3 million people, of whom 1.8 million are displaced. Many of the displaced people fled earlier offensives to escape the fighting or avoid coming back under Assad's rule - including those bussed to Idlib under "evacuation" deals when rebels surrendered other enclaves.
Evacuations from eastern Ghouta, Deraa and other rebel territories captured by the government have contributed to the displacement of more than half a million people to or within Idlib in the past six months, the U.N. said.