Surrounded by regime troops in Aleppo’s eastern neighborhoods since last week, rebel militia groups have launched a sustained attack to drive through government lines to connect with opposition territory in the west.

They also initiated attacks on other government-controlled areas, the UK-based Syrian Observatory for Human Rights reported, though a CNN source said fighting had eased in the city’s west.

The observatory said that Russian aircraft were behind the strikes at dawn.

Russia has carried out airstrikes in Syria since September in support of the regime of Syrian President Bashar al-Assad.

Photojournalist and Aleppo resident Karam Al Masri counted 27 strikes on the area since the morning, and three more barrel bombs dropped as he spoke to CNN in the early evening.

Al Masri said he took cover in a basement for an hour with neighbors, including children, as strikes hammered the area.

Syria Civil Defense, a voluntary rescue group also known as the White Helmets, said in a Facebook post that 10 people had been killed by the strikes.

Al Masri said he witnessed one man die.

“I saw seven others injured. One of them was a child who lost his leg, and he is now in a serious condition. He was just 7 or 8 years old. This happened in my neighborhood, Bustan Al-Qasr. All airstrikes today targeted residential areas,” he said.

Dozens of combatants on both sides were killed, the Syrian Observatory for Human Rights said.

Hospitals supported by Doctors Without Borders in rebel-held Aleppo have seen a significant increase in wounded seeking treatment since the fighting intensified this week, said Marco Pablo Blanco, Middle East operations manager for the humanitarian group.

About 250,000 people are facing a humanitarian crisis in besieged eastern Aleppo since the government cut supply lines to rebel-held areas, the United Nations says.

Heavy fighting has taken place on the outskirts of Ramouseh, a critical district separating the surrounded rebel enclave from rebel-held territory to the west, the Syrian Observatory for Human Rights reported.

Rebels detonated a bomb in a tunnel underneath a government position in the Ramouseh area, the observatory said.

Rebel shelling of government areas have killed 40 people, including 22 women and children, in the past 48 hours, the observatory said.

Rebel groups operating in the city include so-called moderate factions such as the Free Syrian Army as well as an array of Islamist groups, including Jabhat Fateh Al-Sham, formerly known as al Nusra Front, which recently broke its ties with al Qaeda.

Rebels have held the eastern part of Aleppo for about four years.

The Syrian and Russian governments say three humanitarian corridors have been opened to allow for the distribution of badly needed food and medical aid to civilians and to provide residents — along with rebels who choose to surrender — the opportunity to leave.

They say that both civilians and surrendering rebels have used these routes.

But CNN sources on the ground and international observers say the corridors do not appear to be in wide use, and U.S. Secretary of State John Kerry suggested last week that the corridor approach could potentially be a “ruse.”

“It’s my impression that there (are) few people so far going out through those so-called corridors,” Jan Egeland, humanitarian adviser to the U.N. envoy to Syria, told CNN.

The corridors, he said, “need to be guaranteed by all parties in the area.”

Blanco told CNN that none of Doctors Without Borders’ sources in the city had seen the corridors being used.

Rather than corridors guaranteed by Syria and Russia, Egeland said, Aleppo needed a “normal” international humanitarian operation, run by the United Nations, Red Cross and Red Crescent, and a 48-hour pause in the fighting agreed to by all parties.

“If this doesn’t happen, it will be a black stain on the conscience of humanity,” he said.

Referring to the strikes on hospitals and medical facilities on both sides of the front lines, he said, “We have now sunk in the Syrian war to a new low. It’s like we’re erasing a century of progress for humanity, for civilization.”

On Tuesday, Syria’s government claimed “terrorist groups” carried out a gas attack in Aleppo’s old town, killing five people, according to the state-run Syrian Arab News Agency.

That development followed a separate report earlier that cylinders suspected of containing chlorine gas had been dropped in residential areas in Saraqeb, a city about 50 kilometers (31 miles) southwest of Aleppo.

The attack, which caused about 30 cases of breathing difficulties, occurred in the region where a Russian helicopter was downed Monday, causing the biggest single loss of life for Syria’s key military backer since its warplanes started carrying out airstrikes in September 2015.

A doctor in Saraqeb who told CNN he treated some of those affected said their symptoms were consistent with chlorine poisoning.

A senior U.S. official told CNN on Tuesday that the United States was not ready to officially call the incident a chemical attack.

But the official said there was growing confidence it was such an attack, and that chlorine was likely used.

Russia denies a chemical attack took place.