BEIRUT — Syrian rebels and pro-government forces clashed Sunday on several fronts around Aleppo as the country’s military command called on militants to lay down their weapons and evacuate the contested city.

A day after pro-government forces captured the strategic al-Shuqeef hill north of the city, the Britain-based Syrian Observatory for Human Rights monitoring group reported fierce fighting in areas near the hill and in the Bustan al-Basha neighborhood.

The two sides also clashed in Aleppo’s southern Sheikh Saeed neighborhood.

The government’s attempt to penetrate Aleppo’s opposition-held eastern side has been accompanied by a relentless campaign of airstrikes by Russian and Syrian warplanes.

President Bashar Assad’s forces are depending on the Russia bombardment and Iran-backed militias for support.

A spokesman for the Nour el-Din el-Zinki rebel faction told The Associated Press that foreign fighters were actively participating in the government’s ground campaign. He said rebels could identify Lebanese and Iraqi militias by their flags.

An airstrike, meanwhile, targeted a rebel headquarters near the central city of Hama, killing at least six militants, the Observatory said. It was a setback for the rebel campaign to advance on the government-controlled city.

The U.N.’s humanitarian chief, Stephen O’Brien, reported that eastern Aleppo’s health system has been “all but obliterated” by shelling and bombardment.

“Medical facilities are being hit one by one,” O’Brien said in a statement that called for a 48-hour humanitarian pause to the fighting each week. The U.N. estimates 275,000 people are trapped by the government siege.

“We are in a race against time to protect and save civilians in eastern Aleppo city. They need our urgent action to bring an end to their living hell,” O’Brien said.

One of Aleppo’s largest hospitals, located in the eastern Sakhour neighborhood, was knocked out of service Saturday by the airstrikes, doctors and activists reported.

The Syrian military command said in a statement on state media that government forces would guarantee gunmen safe passage out of opposition-held neighborhoods.

Russia announced a month ago that the Syrian government would give safe passage to civilians wanting to leave eastern Aleppo. Few have accepted the offer.

The U.N. says at least 320 civilians have been killed since the government announced its offensive Sept. 22.

The European Union offered to help evacuate patients in Aleppo’s hospitals and deliver food, water and medical aid to besieged eastern districts.

In a statement, EU foreign policy chief Frederica Mogherini urged international players to unite to make the aid effort work “for the sake of humanity and the political future of Syria.”

The EU is mobilizing $25 million in emergency aid and offering to move patients to other medical facilities, including in Europe, if needed.