Syrian opposition forces have launched an offensive aimed at breaking the regime's siege of eastern Aleppo, where the UN estimates some 300,000 people are trapped with dwindling food and medical supplies.

An opposition military command center that includes the newly formed group Islamist Jabhat Fatah al Sham, the former Nusra Front and Ahrar al Sham, said they had taken over army positions in the southwestern regime-held parts of the city in the first few hours of launching the battle to break the siege imposed on opposition-held areas.

The Britain-based Syrian Observatory for Human Rights (SOHR), which gathers information from a network of local informants, said opposition forces and pro-regime forces were clashing along several fronts on the outskirts of the divided city.

Regime forces closed off the last route to the opposition holdout in early July, replicating siege tactics that it has employed with mixed results throughout the war.

The UN's special envoy to Syria, Staffan de Mistura, warned on Friday that basic supplies in eastern Aleppo could run out in three weeks.

Rights groups have warned that it is illegal to deprive civilians of basic necessities, and that residents should not have to choose between leaving their homes or starving.

Hospitals targeted

In southern Syria, an air strike on a hospital in an opposition-controlled town put the facility out of service Sunday.

The hospital in Jasem was targeted in one of several air strikes to hit the town in Deraa province, located some 50km south of Damascus, according to the Local Coordination Committees activist network.

Hospitals are regularly targeted in Syria's war, drawing condemnation from the UN and the international community.

The New York-based Physicians for Human Rights says over 90 percent of attacks on medical facilities in Syria have been carried out by pro-regime forces.

"This is just one of many instances where hospitals have been targeted in Syria. And at some point we have to say this is enough and people have to be held to account," Sanjayan Srikanthan, Deputy Executive Director of the International Rescue Committee, told Al Jazeera.

"Red lines have been declared by the international community and red lines have been ignored."

PHOTO CAPTION

Men transport a casualty at a site hit by airstrikes in the opposition held Douma neighborhood of Damascus, Syria, July 25, 2016 (Reuters).

Al-Jazeera