Three hospitals including a children's hospital in rebel-held eastern Aleppo were take out of operation on Friday, after being repeatedly targeted by Syrian and Russian warplanes, according to activists and aid workers.

Witnesses said that the hospitals were targeted by vacuum bombs and toxic gas on Friday, marking the fourth straight day of bombardment since Russia declared an end to a month-long humanitarian pause on Tuesday.

A spokesperson from the Independent Doctor's Association, which runs eastern Aleppo's only children's hospital, told Middle East Eye that the heavy bombardment had forced its staff to evacuate the medical facility for safety reasons.

Speaking from Turkey where the IDA's offices are based, the spokesperson said that its medical facility and staff had been targeted twice in one week.

Aleppo's health director said in a statement on Friday that every hospital in the eastern part of the city has been destroyed.

"Due to systemic attacks that targeted Aleppo City Hospitals in the last 48 hours by the Syrian regime and Russian regime airforces; We in Free Aleppo directorate annouce that all hospitals in eastern Aleppo city are out of service," he wrote.

Footage taken by Al Jazeera from the hospital showed patients and staff struggling to breath after the medical facility was hit by bombs containing toxic gas.

Three cases of suffocation was also reported by the Al Jazeera team, with many patients filmed wearing breathing masks.

Many hospitals including the children's hospital, were forced to go underground, in an attempt to keep patients safe from aerial and artillery bombardment by Russian and Syrian government forces.

Following the continous bombardment by Moscow and Damascus this week, four out of seven hospitals are still in operation serving about 250,000 civilians living under siege.

Two out of the four hospitals left in operation contained a trauma and intensive care unit.

Other hospitals that were forced out of operation include the M1 hospital which houses a trauma unit.

Earlier this year MEE reported the use of illegal vacuum bombs against hospitals in east Aleppo on the M2 hospital which was taken out of service last month.

Many residents of eastern rebel-held Aleppo spent the night in basements and bomb shelters after one of the most intense bombardments of the renewed offensive.

'It is shocking that healthcare facilities continue to be attacked in Syria'

- WHO spokesman Tarik Jasarevic

Barrel bombs and other ordnance rained down until midnight, only to resume in the early hours, an AFP correspondent reported.

The bombardment came as government troops pressed an assault on the southern neighbourhood of Sheikh Said, which they briefly entered before being pushed back by rebel fighters, the Syrian Observatory for Human Rights said.

Days of intense bombardment

Speaking via WhatsApp, MEE contributor Zouhir Al-Shimale said that living in east Aleppo "felt like hell" after days of continuous bombardment.

Al-Shimale also told MEE that the bombing had not stopped since the morning.

Witnesses described the artillery assault as the heaviest they had experienced in Aleppo in years of war.

"I have never heard such intense artillery bombardments," said Najib Fakhoury, head of the White Helmets volunteer rescue group in the rebel-held Ansari district.

"Earlier, we received a call for help to extinguish a fire," he said. "But we cannot go because the shells are falling on the streets".

Activists and aid workers have refused to name the facilities, despite using codes to denote hospitals in east Aleppo, as they feared further retaliation from forces loyal to the Syrian government.

The World Health Organisation earlier this week condemned attacks against hospital and medical facilities.

WHO spokesman Tarik Jasarevic said: "It is shocking that healthcare facilities continue to be attacked in Syria. Only this year there were 126 attacks on health facilities and health workers recorded by the World Health Organisation and partners."

He also added: "We call on all parties to stop targeting health workers and hospitals because this is outrageous, and this deprives (the) civilian population from access to basic health services."

Russia confirmed on Thursday that it had deployed cruise missiles to target "jihadist groups in Syria".

It said Thursday's strikes targeted the Islamic State (IS) group and the formerly al-Qaeda-affiliated Fateh al-Sham Front, but it did not specify where they had been carried out.

Aleppo province is mostly controlled by a rebel alliance known as the Army of Conquest, which groups rebel factions with Islamic militants of the Fateh al-Sham Front.

Rebels hit back

Rebels responded by firing more than a dozen rockets into government-held areas of the city, the Syrian Observatory for Human Rights added. Three civilians including two children were killed in western Aleppo, according to Syrian state news agency SANA.

At least 65 civilians have been killed since the offensive resumed on Tuesday, the Observatory said.

No aid has entered the city's eastern neighbourhoods since government troops surrounded them in mid-July, and humanitarian organisations said this week food aid stockpiled there had all but run out.

Once Syria's main commercial and industrial hub, Aleppo has been devastated by fighting since the rebels overran the east of the city in 2012.

The 1.2 million civilians living in government-held areas have come under repeated rocket fire from the rebels.