AMMAN (Reuters) - The Syrian army, supported by Russian jets and Iranian-backed militias, escalated bombing on Sunday of areas of the Syrian city of Deir al-Zor still held by Islamic State.

Smoke rises as Syrian army soldiers stand near a checkpoint in Deir al-Zor, Syria September 21, 2017. REUTERS/Omar Sanadiki

Former residents and the Syrian Observatory for Human Rights said there were heavy aerial strikes on eastern Syria’s largest city as troops pushed towards the Hay al Umal area, which overlooks some of the remaining militant-held neighborhoods where an estimated 1,500 civilians are trapped.

The Syrian army has gradually tightened the noose around the militants after it opened a land route into the city in September with the help of Russian air strikes and Iran-backed militias, breaking a siege that had lasted nearly three years.

“The situation is catastrophic, there are families under the rubble and others who fled have no shelter,” said Sheikh Awad al Hajr, a tribal leader, referring to the plight of those remaining inside the city and in cities, towns and farms in the fertile strip along the Euphrates bordering Iraq.

Fighting and relentless air strikes in Deir al-Zor province, the last stronghold of the Islamic State, have prompted tens of thousands of civilians to flee, former residents and aid workers say.

Relatives of some civilians and Syrian opposition figures accuse the Russian army of bombing boats and dinghies carrying families fleeing the western banks of the Euphrates. Moscow denies it targets civilians in its military operations in Syria and says it hits only militant hideouts and facilities.

Russia has thrown its military weight behind the Syrian army campaign to regain the strategic oil-rich province which has become the focus of Syria’s more than six-year long civil war. They are racing with U.S- backed forces to grab territory from Islamic State.

While the Syrian army appeared to make more gains inside Deir al-Zor city, the militants made a surprise offensive in the last 24 hours that pushed back pro-government and Iranian-backed militias from Albu Kamal, the last border post on the Syrian Iraqi border still in militant hands.[L8N1N25IJ]

The Syrian Observatory for Human Rights and former residents said the jihadists recaptured the strategic towns of al Qwaira and Makhan on Saturday in several deadly ambushes that inflicted heavy casualties on pro-government and Iranian Shi’ite militias.

The latest jihadist assault pushed back the army to the city of Mayadeen, further north along the Euphrates river that the militants lost earlier this month. [L8N1MX05W]

“The Islamic State was able to push back the regime and its Iranian-backed militias to the heart of the city of Mayadeen,” said Amer Huweidi, an activist from the city in touch with locals and residents.

Mayadeen is a strategic city that has been a base for the militants after they were driven out of their de facto Syrian capital in Raqqa city.

The U.S.-led coalition against Islamic State is waging a separate campaign against the group in Deir al-Zor, focused on areas to the east of the Euphrates River which bisects the province.

The coalition secured the Omar oilfield, Syria’s largest oilfield, this month.[L8N1MX05W]