At least 49 people have been killed in heavy government air strikes in the eastern part of Syria's largest city, Aleppo, witnesses and activists say.

The overnight bombardment, which began late on Thursday, was part of a wider military escalation by the Syrian government and its allies against opposition groups holed up in Aleppo.

With the latest victims, the total number of people killed in the besieged city since Bashar al-Assad's government launched its military offensive on Tuesday has climbed to 150.

Activist released dramatic video footage of a rescue operation involving a six-year-old child who was trapped under the rubble of a collapsed building.

READ MORE: In east Aleppo 'there is no way out'

The child survived after his residential neighbourhood was targeted with missiles and unguided explosive devices called barrel bombs.

Al Jazeera's Osama Bin Javaid, reporting from Gaziantep on the Turkey-Syria border, said the bombardment was the fiercest of the past three days.

"The bombs struck areas on Aleppo's outskirts as well as in the city itself. The toll keeps mounting by the hour," he said.

"Rescuers are trying to help as many people as possible but because this is such a widespread area, they cannot get to every location."

On Sunday, the Syrian army sent a text message to residents of east Aleppo, demanding they leave areas held by opposition armed groups within 24 hours or risk their lives during a major offensive.

"Our dear people living in east Aleppo, the militants kill your children and take your women," read the text message, which declared the government's intent to retake opposition-controlled districts of the city.

About 250,000 people are believed to be living in besieged east Aleppo, and Syrian government forces have reversed recent gains made by the fighters last month in their effort to break the siege.

Humam al-Malah, a member of the Syrian Network for Human Rights in the Aleppo governorate, told Al Jazeera that humanitarian conditions are getting worse in east Aleppo.

"Electricity is always cut off; [there's a] high increase in prices; an acute lack of vegetable availability; fuel is almost non-existent in markets; and the quality and quantity of supply of bread is dwindling," said Malah.

Against this backdrop, Russia's foreign minister has flatly denied that his country's forces participated in the attacks on Aleppo this week.

Sergey Lavrov issued the denial while discussing the bombing of the city on the sidelines of the Asia-Pacific trade summit in Lima, Peru.

Russia is a crucial ally of the Syrian government and its military has been targeting Syrian opposition fighters with air strikes and cruise missiles.

Lavrov portrayed the recent strikes in Syria as "limited".

"Our air force and the Syrian air force only work in the provinces of Idlib and Homs, to prevent ISIL who might be leaving Mosul from getting to Syria," he said, referring to the Islamic State of Iraq and the Levant (ISIL) group, also known as ISIS.

Artillery shelling

Elsewhere in Syria, in Ghouta in the Damascus countryside, at least 10 people were killed and dozens injured in air strikes and artillery shelling by government forces.

Residents reported substantial damage to residential areas in the town.

Also in the Damascus countryside, the Syrian government targeted an international relief agency and Palestinian refugee camp in Khan Sheha, which has been under siege for two months now.

Syrian forces and opposition fighters fought around the camp on Thursday night.</s>BEIRUT — Doctors and nurses at a pediatric hospital in eastern Aleppo scrambled Friday to evacuate babies in incubators to safety from underground shelters after the facility in the besieged Syrian city was bombed for the second time this week.

Medics and aid workers also reported a suspected attack involving toxic gas in a district on the western edge of the rebel-held area. At least 12 people, including children, were treated for breathing difficulties, said Adham Sahloul of the Syrian American Medical Society, which supports health facilities in Aleppo.

Claims of toxic gas attacks are common in Syria, and reports by international inspectors have held the government responsible for using chemicals in attacks on civilians, which Damascus denies.

Friday was the fourth day of renewed assaults by Syrian warplanes on eastern Aleppo districts, a rebel-held enclave of 275,000 people. The onslaught began Tuesday, when Syria's ally Russia announced its own offensive on the northern rebel-controlled Idlib province and Homs province in central Syria.

Friday's airstrikes in Aleppo hit a complex of four hospitals that had been attacked two days earlier. The latest strikes forced the pediatric hospital and a facility to stop operating.

"Now it is being bombed. ... I am sorry. ... I have to go to transfer the children," the head of the pediatric hospital wrote in a text message to The Associated Press. The doctor identified himself only by his first name of Hatem because he fears for reprisals against his family.

The incubators already had been moved underground for safety, but with bombs falling all around the facility, hospital workers had to rush them to a safer place despite the danger.

"As we drove out with the ambulance, warplanes were firing and artillery were shelling," he wrote. "But thank God we were not hurt."

The cameras of Al-Jazeera, which was broadcasting from the facility as the airstrikes occurred, went dark for a moment. When video resumed, dust was swirling and debris was strewn in the corridors.

Nurses scurried to get babies to safety, and one was seen carrying a blanket-wrapped infant. She then hugged and comforted another nurse who was sobbing as she picked up a baby.

Another hospital in a different Aleppo was bombed Thursday night, the doctor told AP. The entrance was set on fire but no one was hurt.

Government bombings have targeted with medical facilities, including the children's hospital and a nearby clinic that has one of the few remaining intensive care units in eastern Aleppo, the Observatory said.

Many hospitals and clinics in the besieged area have moved their operations underground after months of relentless bombardment. The World Health Organization said that in 2016, it recorded 126 attacks on health facilities, a common tactic over the five years of Syria's civil war. Russia and the Syrian government deny targeting hospitals.

Ibrahim al-Haj, a member of the Syria Civil rescue unit in Aleppo, said the city "is a mess." The group of rescuers and first responders said they are struggling to put out fires set off by the bombings in at least 10 areas.</s>BEIRUT -- Doctors and nurses at a pediatric hospital in eastern Aleppo scrambled Friday to evacuate babies in incubators to safety from underground shelters after the facility in the besieged Syrian city was bombed for the second time this week.

Medics and aid workers also reported what was believed to be an attack involving toxic gas in a district on the western edge of the rebel-held area. At least 12 people, including children, were treated for breathing difficulties, said Adham Sahloul of the Syrian American Medical Society, which supports health facilities in Aleppo.

Claims of toxic-gas attacks are common in Syria, and reports by international inspectors have held the government responsible for using chemicals in attacks on civilians, which Damascus denies.

Airstrikes also hit a village in a rural area of Aleppo province, killing seven members of a family, including four children, opposition activists said.

Friday was the fourth day of renewed assaults by Syrian warplanes on eastern Aleppo districts, a rebel-held enclave of 275,000 people. The onslaught began Tuesday when Syria's ally Russia announced its own offensive on the northern rebel-controlled Idlib province and on the Homs province in central Syria.

Since then, more than 100 people have been killed across northern Syria.

Friday's airstrikes in Aleppo hit a complex of four hospitals that had been attacked two days earlier. The latest strikes forced the pediatric hospital and a neighboring facility to stop operating.

"Now it is being bombed. ... I am sorry. ... I have to go to transfer the children," the head of the pediatric hospital wrote in a text message. The doctor identified himself only by his first name of Hatem because he fears reprisals against his family.

The incubators already had been moved underground for safety, but with bombs falling all around the hospital, workers had to rush them to a safer place despite the danger.

Hatem rushed 14 babies in incubators to another facility a 10-minute drive away while airstrikes continued, he said in a later message.

"As we drove out with the ambulance, warplanes were firing and artillery were shelling," he wrote. "But thank God we were not hurt."

Some of the survivors of the suspected gas attack were taken to the children's hospital.

Images from Al-Jazeera, which was broadcasting from the facility as the airstrikes occurred, went dark for a moment. When video resumed, dust was swirling and debris was strewn in the corridors.

Nurses scurried to get babies to safety, and one was seen carrying a blanket-wrapped infant. She then hugged and comforted another nurse who was sobbing as she picked up a baby.

Another hospital in a different Aleppo neighborhood was bombed Thursday night, the doctor said. The entrance was set on fire, but no one was hurt.

Only four of seven hospitals are still operating in the district, Sahloul said.

The Britain-based Syrian Observatory for Human Rights said dozens of airstrikes, artillery and barrel bombs hit 18 different neighborhoods of eastern Aleppo.

Government bombings have targeted neighborhoods with medical facilities, including the children's hospital and a nearby clinic that has one of the few remaining intensive care units in eastern Aleppo, the Observatory said.

Many hospitals and clinics in the besieged area have moved their operations underground after months of relentless bombardment. The World Health Organization said that in 2016, it recorded 126 attacks on health facilities, a common tactic over the five years of Syria's civil war. Russia and the Syrian government deny targeting hospitals.

The city of Aleppo, once Syria's commercial hub, has been divided since 2012, with the eastern half in rebel hands and the western half controlled by government forces.

Ibrahim al-Haj, a member of the Syria Civil Defense rescue unit in Aleppo, said the city "is a mess." The group of rescuers and first responders said they are struggling to put out fires set off by the bombings in at least 10 areas.

The Observatory said at least four people were killed in the city's districts.

It also said the strike that killed the seven family members took place in southwestern Aleppo province. Syrian Civil Defense posted photos showing the bodies of children covered with dust and blood.
Media reports say a hospital in Aleppo neighborhood is bombed on night by the Syrian government.