ISTANBUL — Syrian government and allied Russian warplanes on Monday intensified a week-long bombardment of Idlib province, targeting hospitals and other civilian infrastructure as tens of thousands of residents fled toward the border with Turkey, activists and monitors in the rebel-held region said.

The aerial campaign has killed about 100 civilians and put at least 10 hospitals out of service. It has raised fears that Syrian government forces, supported by Russia and Iranian-backed fighters, are preparing an all-out offensive in Idlib — the last area in the country controlled by rebels opposed to President Bashar al-Assad.

The airstrikes represent the latest and fiercest challenge to a pact brokered by Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan last year that was designed to avert all-out conflict in the northwestern province.

A radio station based in the city of Kafr Nabl in Syria's Idlib province uploaded video May 5 that reportedly showed the aftermath of a Russian airstrike. (Radio Fresh via Storyful)

On Monday, government helicopters dropped 13 barrel bombs in southern areas of Idlib and northern pockets of neighboring Hama province while Russian jets carried out at least 33 airstrikes in the region, according to the Britain-based Syrian Observatory for Human Rights.

The uptick in hostilities has prompted an exodus of about 200,000 of Idlib’s 3 million residents toward the sealed Turkish border, in what the Union of Medical Care and Relief Organizations, a U.S.-based aid group, has described as the beginning of an “Apocalyptic scenario.”

“The southern countryside of Idlib is almost empty because of the destructive shelling that targeted the whole area,” said a 38-year-old activist from Idlib who asked to be identified by the nickname Abu Muhammad. “It is brutal what is happening.”

Abu Muhammad and others in the sprawling province said they expect the government and allied Iranian militias to soon launch a ground assault, which would mark the beginning of the final and largest battle waged by Assad to wrest back territory lost to rebels since the start of Syria’s civil war.

It would also spell the breakdown of the Russian-Turkish agreement that took hold in October. The cease-fire called for a nine-mile demilitarized zone in Idlib. It has been repeatedly violated by both sides, but the Syrian Observatory for Human Rights has described the latest escalation as the “fiercest.”

The European Union and the United Nations last week called on Russia and Turkey to preserve the cease-fire, while the State Department issued a statement warning that the situation threatens to destabilize the region.

“We call on all parties, including Russia and the Syrian regime, to abide by their commitments to avoid large scale military offensives, return to a de-escalation of violence in the area, and allow for unhindered humanitarian access to address the humanitarian disaster created by the ongoing violence,” State Department spokesman Morgan Ortagus said in the statement.

Syrian officials quoted in state media have said the operations in Idlib are in response to attacks by al-Qaeda-linked militants that killed 22 government troops and allied militiamen late last month.

Idlib province is dominated by Hayat Tahrir al-Sham, or HTS, a hard-line group affiliated until recently with al-Qaeda.

Turkey-backed Syrian rebels in Idlib said the intensifying airstrikes are probably tied to a growing economic crisis in government-controlled areas, driven by U.S. and E.U. sanctions.

According to rebels interviewed by Reuters, Assad and Russia are keen to reclaim two key highways that link Idlib to major cities controlled by the government and serve as vital commercial routes between Syria’s borders with Jordan in the south and Turkey in the north.

Abu Muhammad, the activist in Idlib, said harsh economic conditions — including fuel shortages in Damascus, the capital — are provoking Assad and Russia to project strength through military action.

“Regime areas are suffering from a huge economic crisis,” he said. “Civilians in regime-controlled areas have started to complain about it. . . . So the regime wants to distract everyone with this battle.”