Syrian opposition forces backed by US airstrikes lifted a siege Wednesday on the frontline town of Marea in northern Aleppo province, pushing back Islamic State fighters, a monitoring group said.

The rebels appear to have been helped by the wider offensive led by the Kurdish-dominated Democratic Forces of Syria (DSF) further to the east, which is placing heavy pressure on the Islamist militants.

The DSF offensive is focused on capturing territory in the so-called Minbij pocket, Islamic State's last stretch of land along the Turkish border. Minbij city lies just 20 kilometres from the frontier. DSF forces say they are on the city's outskirts.

"The siege has been lifted on Marea," Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told dpa.

Marea is a key part of a rebel-held enclave in Aleppo, a shrinking piece of territory near the border with Turkey.

In May, Islamic State attacked Marea, cutting it off from the lifeline of A'zaz on the Turkish border. Some 160,000 people are trapped in the area as Turkey has shut its border to civilians trying to flee.

The northern Aleppo enclave was cut off from the city to the south in February by government forces, backed by Russian air power.

The government is continuing to batter southern Aleppo, launching a barrage of airstrikes on rebel-held eastern Aleppo city. Helicopters were dropping highly inaccurate and destructive barrel bombs on the city, activists said.

At least 15 people were killed and as many injured in the attacks, according to Abdel Rahman.

In Aleppo city, video footage taken by activists on the ground showed heavy damage to buildings. Activist Mahmoud al Shahabi told dpa that two hospitals in the area sustained some damage.

UNICEF in statement said three medical facilities were attacked Wednesday in Aleppo within three hours.

"The al-Bayan and al-Hakeem hospitals, within a distance of 300 metres, and the Abdulhadi Fares clinic, all in the eastern part of the city were hit," it said.

The Al Hakeem hospital is a UNICEF supported facility and one of the few that still provide pediatric services.

"Everyone must question their humanity when babies have to be taken out of incubators because of attacks on hospitals," the statement said.

Shahabi, who lives in Aleppo, said cemeteries are full and people are using public garden and playgrounds to bury loved ones.

The city is split, but the rebel-held section is coming under increasing pressure. The Syria Campaign, a pro-opposition advocacy group, said in a tweet: "Aleppo has run out of coffins and body bags. And the bombs keep falling."

President Bashar al-Assad this week vowed to step up attacks on "terrorists," a term he employs to refer to those opposing his rule.

In Washington, State Department spokesman Mark Toner said al-Assad was "sadly mistaken if he thinks there's a military solution in Syria."

The US condemned the attacks on the Aleppo hospitals, Toner said: "The fact that the regime is once again targeting medical facilities is unconscionable."

Observers said Islamic State forces retreated from Marea and nearby villages in order to shore up their lines in Minbij, a major transit artery near to al-Raqqa, the extremists' de-facto Syrian capital.

Kurdish officials claim the extremists still have supply lines running over the Turkish frontier into the Minbij area.

The United States, which has launched more than 100 airstrikes in Minbij and Marea to help the ground forces, says the goal of the operation is to limit foreign fighter flows to Islamic State.

Turkey has been wary of the offensive against Islamic State, due to close links between the group at the core of the DSF, the Kurdish People's Protection Units (YPG), and the Kurdistan Workers' Party (PKK), which has been fighting Turkish security forces in south-eastern Turkish provinces.

Turkish Foreign Minister Mevlut Cavusoglu said he was given promises by the US that Kurdish forces would not remain in Minbij after Islamic State was ousted. Washington has not confirmed Cavusoglu's claim.

The defence ministers of Iran, Russia and Syria are scheduled to meet Thursday in Tehran to discuss the battle against Islamic State, according to Iranian media reports.

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