Syrian regime forces shelled rebel and extremist positions in the north-western province of Idlib on Thursday and dropped leaflets warning of an impending assault.
The province is the largest chunk of Syrian territory still in rebel hands, and President Bashar Al Assad has warned it would be his military's next priority.
The United Nations appealed on Thursday for talks to avert "a civilian bloodbath" in Idlib.
"The war cannot be allowed to go to Idlib," the head of the United Nations humanitarian taskforce for Syria, Jan Egeland, said in Geneva.
Mr Egeland said he remained "hopeful" that diplomatic efforts under way could avert a major ground offensive that would force hundreds of thousands to flee.
"It is bad now" in Idlib, Mr Egeland said. "It could be 100 times worse."
The warning came as government helicopters dropped leaflets over towns in Idlib's eastern countryside urging people to surrender.
"The war is nearing an end ... We are calling on you to join the local reconciliations, as many of our people in Syria did," said the leaflets, which were stamped with the military's seal.
Such surrender deals have typically seen rebels hand over territory to government troops in exchange for a halt to shelling, the return of state institutions, and a chance to either join regime forces or be transported to other rebel-held areas.
"The fate of your family, children, and future depend on your decision," warned the leaflets.
Heavy artillery and rocket fire on Thursday morning hit territory around Jisr Al Shughur, a key town in the south-western part of the province, according to the Syrian Observatory for Human Rights.
"The shelling is in preparation for an assault but there has been no ground advance yet," said Rami Abdel Rahman, head of the UK-based war monitor.
"Regime reinforcements including equipment, soldiers, vehicles and ammunition have been arriving since Tuesday," he said.
They were being distributed along three regime-held fronts, including in neighbouring Latakia province just west of Jisr Al Shughur, in the Sahl Al Ghab plain south of Idlib, and in a sliver of the province's south-east that is already in government hands.
Al Watan newspaper, which is close to the government, reported that army troops had also bombed rebel and extremist positions in the area.
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Idlib, which has been outside of regime control since 2015, lies along the border with Turkey but is otherwise nearly completely surrounded by government-held territory.
About 60 per cent of the province is now held by Hayat Tahrir Al Sham (HTS), which is led by Al Qaeda's former Syria affiliate, while the rest is controlled by rival opposition factions.
Syrian troops have recaptured key areas of the country in recent months with help from ally Russia, which has brokered a string of surrender deals with rebels.
Apparently fearing a similar arrangement for Idlib, HTS has been arresting dozens of figures in the province who have been go-betweens with the regime.
Early on Thursday, the group detained several such figures from villages in Idlib's south-east, calling them "chiefs of treason", according to an HTS-linked media agency.
The Observatory, which relies on a network of sources inside Syria, said it had documented more than 100 such arrests by HTS and rival forces this week alone.
Idlib province is home to about 2.5 million people, including rebels and civilians transferred en masse from other territory that fell to Syrian troops after intense assaults.
It was designated last year as one of four "de-escalation" zones where violence was supposed to be reduced ahead of a nationwide ceasefire.
It is the only such zone left, after Mr Al Assad's troops in recent months recaptured the other three with a mixture of military assaults and "reconciliation" deals.
