BEIRUT (Reuters) - Syria and its ally Russia have stepped up an offensive against the last big stronghold of Syrian rebels, mounting more air raids and deploying ground reinforcements including Iranian-backed militias, army defectors and residents said on Friday.

Syria Civil Defence (White Helmets) members search the rubble of a damaged building for victims, after a deadly airstrike, said to be in Maarat al-Numan, Idlib province, Syria August 28, 2019. Syria Civil Defence in the Governorate of Idlib/Handout via REUTERS

The Russian-led alliance is pushing into densely populated parts of Idlib province in the northwest where millions of people who fled fighting elsewhere in Syria have taken refuge.

The northwest offensive has prompted U.N. warnings of a new humanitarian crisis amid the gains by Damascus and its partner Moscow, which has helped President Bashar al-Assad turn the tide in the eight-year-old conflict since intervening in 2015.

Moving deeper into territory along the Turkish border, the advance took the town of Tamaneh after earlier capturing Khwain, Zarzoor and Tamanah farms, the defectors and residents said.

They were the first gains since the alliance, battling a coalition of jihadists and mainstream Turkey-backed rebels, seized a main rebel pocket in nearby Hama province last week.

The offensive has been reinforced by elite army units and Iranian-backed militias, the defectors and residents said.

“There are daily reinforcements coming from the Iranian militias, elite Republican Guards units and Fourth Armoured Division,” Colonel Mustafa Bakour, a commander in Jaish al Izza rebel group, told Reuters.

Jets flying at high altitude dropped bombs on the outskirts of Idlib city, the heavily-populated provincial capital. The aircraft were believed to be Russian, according to activists who track the warplanes’ activities.

Russia’s defense ministry said Syrian government forces will unilaterally cease fire in the “de-escalation zone” in Idlib region on Saturday morning, TASS news agency said. [R4N25M04O]

Turkey, Russia and Iran agreed in 2017 to make Idlib a de-escalation zone to reduce fighting, although the terms were never made public and the deal did not include jihadist groups.

Foreign Minister Sergei Lavrov said earlier this month that Russia had military personnel on the ground in Idlib province. [R4N25A021]

In recent weeks, Moscow sent more special forces that helped break months of stalemate on frontlines where rebels had been holding the army back, Western intelligence sources have said.

AIR STRIKES

Rebel resistance has been eroded by relentless air strikes against civilian areas since the advance began in late April. The campaign has destroyed dozens of hospitals, schools and civil defense centers, paralyzing life in rebel-held areas.

Moscow and Damascus deny they have targeted civilians and say they are responding to militant attacks by the former Nusra Front, a jihadist alliance now known as Hayat Tahrir al-Sham that is the dominant force in Idlib.

Opposition sources say hundreds of troops from the country’s elite Republican Guards, which is led by President Bashar al Assad’s brother Maher al-Assad, have been deployed on the frontlines of southern Idlib province.

The rapid progress of the last few weeks has been attributed to the new lineup of Russian backed-forces, an army defector and two senior opposition sources conceded.

“The Russians have now moved to depending on the Iranians and elite army formations in this campaign,” Bakour added, saying this was a move away from reliance on the so-called Tiger forces who previously provided most of the army’s ground troops.

Speaking in Oslo, Turkey’s Foreign Minister Mevlut Cavusoglu said Russia had assured Turkey its observation posts in northwest Syria would not be attacked.

Hundreds of angry Syrians protesting against the Russian and Syrian bombing demonstrated near the main Bab al Hawa border crossing with Turkey, demanding Ankara’s intervention to stop the Russian backed offensive. Turkish security forces used tear gas to disperse demonstrators who sought to enter the Turkish side of the border, witnesses said.

Syrian troops taking part in the offensive have encircled rebels and a Turkish military observation post near the town of Morek. The post is one of 12 that Ankara established in the northwest under a deal with Moscow and Tehran two years ago to reduce fighting between Assad’s forces and rebels.

Since capturing the strategic town of Khan Sheikhoun nearly 10 days ago, Russian and Syrian jets were now escalating strikes on the city of Maraat al-Numan that lies further north.

At least 12 civilians, including five children, were killed in raids on the city, where most of the inhabitants have fled.

More than half a million civilians have now been uprooted in the course of the offensive. The United Nations says hundreds of civilians have been killed in the violence.