Russia has military servicemen stationed on the ground in Syria's region of Idlib and is following the situation there closely, Interfax news agency cited Foreign Minister Sergei Lavrov as saying on Tuesday.
Lavrov was quoted as saying that any attacks carried out by Islamist militant groups in the de-escalation zone in Idlib would be forcefully suppressed. Russian military forces have long been operating in the area.
Syrian government forces look set to recover a strategic town that has been in rebel hands since 2014 in a major Russian-backed offensive into the opposition's last major stronghold.
An organisation that monitors the war and a pro-Damascus military source said insurgents had withdrawn from Khan Sheikhoun overnight, though the main insurgent group in the area said rebels still held part of the town and fighting continued.
Capturing Khan Sheikhoun would be an important gain for President Bashar al-Assad into the northwestern region where his bid to recover "every inch" of Syria has run into complications including Turkish forces on the ground.
Syrian state media, in a broadcast from near the town, reported that government forces had widened their control including by seizing a highway running through Khan Sheikhoun, which was targeted in a sarin poison gas attack in 2017.
The pro-Damascus military source told Reuters the town was under army control after the rebels were caught in a pincer movement and fled.
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"There are some pockets and explosive devices, there are a few who refuse to withdraw and want to die," the source said.
The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said rebels had withdrawn from their last piece of territory in neighbouring Hama province in addition to Khan Sheikhoun.
The most powerful insurgent group in the area, the jihadist Hayat Tahrir al-Sham, denied this and said the battle continued.
In a statement on its Telegram channel, Hayat Tahrir al-Sham said rebels still held part of Khan Sheikhoun and nearby areas in northern Hama despite what it described as a redeployment in the town after fierce enemy bombardment.
Another rebel alliance, the Turkey-backed National Liberation Front, said that as a result of bombardment in the northern Hama area its forces had left positions that had become difficult to supply and had redeployed to others.
Rebel fighters "did not leave an inch of land before making the enemy taste great calamities", it said in a statement.
The Observatory and the pro-Damascus military source said Syrian rebels had pulled back to a Turkish military position in the town of Morek, south of Khan Sheikh in Hama province.
Turkey, which supports some of the rebel groups in the northwest, has established a dozen military positions in the area under deals with Russia.
One of its main concerns is to prevent a further influx of Syrian refugees fleeing government control. Some 3.6 million Syrian refugees already live in Turkey.
The pro-Damascus source said negotiations were underway between Turkey and Russia over the withdrawal of the Turkish position.
Turkish Foreign Minister Mevlut Cavusoglu said Turkey would not remove the observation post located near Morek, which a Turkish convoy was trying to reach on Monday when it came under attack.
Syrian government forces stepped up military operations against the northwestern region in late April, in an offensive that has killed hundreds of people and forced hundreds of thousands of people to flee toward the Turkish border.
After eight years of conflict, Assad has defeated his rebel opponents across much of the country with help from Russian and Iran-backed forces.
Russia has military servicemen stationed on the ground in the Idlib region and is following the situation there closely, Interfax news agency cited Foreign Minister Sergei Lavrov as saying on Tuesday. He said any attacks carried out by militants in Idlib's de-escalation zone would be forcefully suppressed.