The Syrian army dropped leaflets over Idlib province on Thursday, urging people in part of the last big swathe of territory still held by rebels to agree to a return of state rule, telling them the seven year war was nearing its end.
Northwestern Syria, where Idlib is located, is the last big area still in the hands of fighters seeking to overthrow President Bashar al-Assad, after the government recaptured the area around Damascus and the southwest earlier this year.
Hundreds of thousands of opponents of Assad have relocated to the northwest under evacuation agreements reached as other parts of the country fell to pro-government forces backed by Russia and Iran. Assad has indicated it could be his next target.
The Syrian Observatory for Human Rights said additional government forces were arriving for a possible attack in an area to the southwest of Idlib city that overlaps with Latakia and Hama provinces.
The United Nations worries that an offensive in the area could force 2.5 million people towards the Turkish border.
Nato member Turkey, which has troops at 12 bases in Syria's northwest to observe a truce agreed with Russia and Iran, has warned against any offensive in Idlib, and is pressing Russia to make sure this doesn't happen.
UN humanitarian adviser Jan Egeland said on Thursday that Turkey, Russia and Iran had all agreed to "do their utmost to avoid" a battle in Idlib.
"This war must end not in a bloodbath but in agreements," he said. But he added that the UN was making preparations and that he would ask Turkey to keep its borders open for fleeing civilians.
"There is intensive diplomatic activity with Russia, Turkey, Iran, the Syrian government and armed opposition groups to avoid escalation in a de-escalation zone, but of course there is conflict every single day indeed there," he added.
The leaflets, dropped in rural valleys near Idlib city, told residents: "Your cooperation with the Syrian Arab Army will release you from the rule of militants and terrorists, and will preserve your and your families' lives."
"We call upon you to join local reconciliation (agreements) as many others in Syria have done," said a leaflet in the name of the army command, a copy of which was seen by Reuters.
Such agreements, concluded at the local level, have been a tool for helping the Syrian government to reestablish control over numerous areas and have often been agreed when rebel fighters are on the brink of military defeat.
The government says the agreements grant an amnesty to rebels who are willing to live under state rule again, unless private lawsuits have been brought against them. The terms also include that they give up weapons.
But many rebels, civilian dissidents and others have instead opted to take safe passage to the opposition-held northwest, an arc of territory at the Turkish border that stretches from Idlib to the city of Jarablus on the Euphrates River.
Idlib is controlled by an array of insurgent groups, with jihadists believed to be the dominant force there.