The Syrian regime warned people in the rebel-held Idlib province the war was nearing its end in leaflets dropped over the northwestern region.
Russia, Iran and Turkey told a meeting of the Syria humanitarian task force on Thursday that they would do their utmost to avoid a battle that would threaten millions of civilians in rebel-held Idlib, UN humanitarian adviser Jan Egeland said.
He estimated that there were four million or more people in the potential battleground in northwest Syria and he hoped diplomats and military envoys could reach a deal to avoid a "bloodbath." But he said the UN was making preparations for a battle and would ask Turkey to keep its borders open to allow civilians to flee if the need arose.
Idlib, at the Turkish border, is one of the last major rebel strongholds in Syria. Regime leader Bashar al Assad, who has defeated rebels across much of Syria with Russian and Iranian help, has indicated it could be his next target.
The Syrian Observatory for Human Rights said additional regime forces were arriving for a possible attack in an area to the southwest of Idlib city that overlaps with Latakia and Hama provinces.
Syrians have fled to the Idlib province from other parts of the country as the regime has advanced, and the United Nations has warned that an offensive there could force 2.5 million people towards the Turkish border in the event of an offensive.
NATO member Turkey has warned against any offensive in Idlib, and is pressing Russia to make sure this doesn't happen. Turkey has established 12 military observation posts in the northwest under an agreement with Russia and Iran.
Airborne leaflet propaganda
Syrian regime forces urged people in rebel-held Idlib province to agree to a return of state rule and told them the war was nearing its end in leaflets dropped over the northwestern region on Thursday.
"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," declared the leaflets that were dropped in rural areas near Idlib city.
"We call upon you to join local reconciliation (agreements) as many others in Syria have done," said the leaflet.
Such agreements, concluded at the local level, have been a tool for helping the Syrian regime to re-establish control over numerous areas and have often been agreed when rebel fighters are on the brink of military defeat.
The regime 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.
Regime shells rebel positions
On Thursday morning, artillery and rocket fire slammed into territory around Jisr al Shughur, a key town in the southwestern part of the Idlib 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 Observatory head Rami Abdel Rahman.
"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 that lies south of Idlib, and in a sliver of the province's southeast that is already in regime's hands.
The Al Watan daily, which is close to the regime, also reported on Thursday that regime forces had bombed rebel positions in the area.
Idlib, which has escaped regime control since 2015, lies along the border with Turkey but is otherwise nearly completely surrounded by regime-held territory.
Around 60 percent of it 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 regime forces have recaptured key swathes 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 that have been go-betweens with the regime.
Early on Thursday, the group detained several such figures from villages in Idlib's southeast, calling them "chiefs of treason," according to a HTS-linked media agency.
The Britain-based 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.