Syria's army and loyalist militiamen Sunday ousted the Islamic State group from its last urban stronghold in the country as regime strikes claimed more lives in a rebel-held enclave near Damascus.

The army said it had taken full control of Albu Kamal in the eastern province of Deir Ezzor, which has changed hands several times.

Government forces announced the capture of the town near the Iraqi border earlier this month but lost it to a blistering IS counter-attack a week ago.

"Syrian troops and allied forces took full control of Albu Kamal, and are removing mines and explosives left by IS," a military source in Deir Ezzor told AFP on Sunday.

"IS put up fierce resistance and tried to use explosives and suicide bombers, but besieging the city allowed the army to clinch the offensive and take full control of the city."

State news agency SANA also reported the advance in Albu Kamal, saying the "Syrian army and its allies eliminated the last Daesh (IS) terrorist pocket in the town".

A string of territorial defeats across northern and eastern Syria had left Albu Kamal as the last significant Syrian town held by IS.

Syria's army announced on November 9 it had ousted IS from the town, but the jihadists retook it in a lightning offensive.

A week later, the army and allied Iraqi, Lebanese, and Iranian fighters broke back into Albu Kamal and steadily advanced through the town.

The Britain-based Syrian Observatory for Human Rights, which has a network of sources across Syria, confirmed on Sunday that regime troops and their allies had captured Albu Kamal. 

"IS fighters withdrew from the city towards the Euphrates River," Observatory head Rami Abdel Rahman told AFP.

"There is no more fighting in the town, but there are clashes around Albu Kamal," he said, adding that Lebanon's Hezbollah, Iran's elite Revolutionary Guards and Iraqi militias had fought alongside regime troops, backed by Russian air power.

The monitor said more than 80 fighters were killed in the three days of ferocious combat to retake the town -- 31 pro-regime forces and at least 50 IS jihadists.

IS seized large areas of both Syria and neighbouring Iraq in a lightning 2014 campaign, but this year has lost much of the territory it once held.

The loss of Albu Kamal caps the group's reversion to an underground guerrilla group with no urban base.

- Heavy bombing near Damascus -

On another front near Damascus, intense regime bombardment on the rebel-held Ghouta region killed eight civilians and wounded 25, the Observatory said, as the United Nations urged all sides to avoid targeting non-combattants.

Six days of heavy bombing have killed dozens in the region, where rebel groups and regime forces are locked in a cycle of tit-for-tat attacks.

On the sixth consecutive day of heavy bombing, an AFP videographer at a hospital in Ghouta's largest city, Douma, saw a man crying as he hugged an unconscious teenager.

A nurse helped a distraught woman who appeared to be searching for a loved one and a man kissed the body of a child wrapped in a shroud.

Doctors struggled to save a child with a blood infusion to the chest as a boy moaned in pain on another hospital bed.

President Bashar al-Assad's forces have besieged Eastern Ghouta since 2013, making humanitarian conditions in the area, where some 400,000 people live, extremely dire.

On Sunday, the UN's coordinator for humanitarian and development affairs in Syria, Ali al-Zaatari, called on "all warring sides to avoid targeting civilians".

"For days, there have been daily reports about civilians being killed and others being severely wounded, in addition to warehouses, hospitals and schools being put out of service during the exchange of shellfire, particularly in Damascus and Eastern Ghouta," Zaatari said.

More than 330,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests. Millions have been displaced.

Diplomats from Iran, Russia and Turkey met Sunday in Antalya to discuss Syria ahead of a three-way summit in the Russian city of Sochi on Wednesday.

Russian Foreign Minister Sergei Lavrov called the meeting with his Turkish counterparts Mevlut Cavusoglu and Iran's Mohammad Javad Zarif "very productive" and said they had agreed on "all the main issues".

Moscow, Tehran and Ankara are sponsoring the so-called Astana peace talks, named for the Kazakh capital where they are regularly held, which calls for "de-escalation" zones in key areas of Syria -- including the Eastern Ghouta region.