Russia denies involvement as toll since start of Assad’s offensive on city’s besieged east climbs to 150.

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.”

Activists released a video of a rescue operation in east Aleppo [Al Jazeera]

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.