Video report by ITV News International Affairs Editor Rageh Omaar
Video part courtesy of Barada TV
The UN has called for an immediate suspension of "all war activities" in eastern Ghouta where more than 270 people have reportedly died after three days of intense bombardment by the Syrian government.
UN Secretary-General Antonio Guterres said that 400,000 people are living "in hell on earth" in the rebel-held suburbs of the capital Damascus.
Speaking at the UN Security Council on Wednesday, Mr Guterres said: "This is a human tragedy that is unfolding in front of our eyes and I don't think we can let things go on in this horrendous way."
At least 276 people, including 50 children, are believed to be among those killed in eastern Ghouta in the shelling by forces loyal to president Bashar al-Assad, which left as many as a 1,000 injured.
Hospitals are now reportedly running at 60% reduced capacity after 22 medical centres were damaged during the bombing raids.
Doctors are now having to turn seriously injured patients away or stop treating them where they are overrun.
Mr Guterres said a suspension of fighting must allow for humanitarian aid to reach all in need and the evacuation of some 700 people needing urgent medical treatment.
He welcomed efforts by Sweden and Kuwait who are drafting a Security Council resolution on the humanitarian crisis in Syria calling for a 30-day cease-fire.
The Syrian Civil Defence - also known as the White Helmets - said a further 10 people had been killed in a targeted attack on Kafr Batna on Wednesday morning.
It is thought the airstrikes laying the ground for determined push to recapture territory which has been controlled by the opposition forces since 2012.
The UN human rights office said in a statement that at least 346 people have been killed in the suburb since the Syrian government escalated their offensive on February 4.
It added another 878 have been wounded, mostly in airstrikes hitting residential areas and 92 of these civilian deaths allegedly occurred in just one 13-hour period on Monday.
It said the numbers are far from comprehensive, documented in the midst of the chaos and destruction.
Theresa May said she was "appalled" by the escalation of airstrikes and "deeply concerned" by reports of the "ongoing targeting" of civilians and called on Assad's regime to allow the hundreds of injured access to aid.
Speaking at Prime Minister's Question Time, Mrs May said: "This is in blatant violation of international humanitarian law and human rights law."
The prime minister said she supported the UN envoy's work in trying to find a political solution to ending the war in Syria, urging backers of Assad's regime, including Russia, to stop the violence.
The Kremlin rejected allegations the Russian military is responsible for civilian casualties in rebel-held suburbs.
Russian President Vladimir Putin's spokesman Dmitry Peskov said Wednesday such claims are "unfounded."
The eastern Ghouta suburbs are home to some 400,000 people as well as thousands of insurgents belonging to different factions. The most powerful is the ultra conservative Army of Islam and Failaq al-Rahman, with a small presence of al-Qaida-linked fighters.
Videos showed paramedics pulling out the injured from the rubble while others are seen frantically digging through the debris in the dark in search of survivors.
Ahmad al-Dbis, an official at the Union of Medical Care and Relief Organizations, which runs hospitals and clinics in Syrian opposition areas, says 10 hospitals have been damaged by airstrikes or shelling in the last 48 hours.
"Yesterday we had more than a hundred martyrs. More than 600 wounded. Today, now we are in the midday, we have more than 80 martyrs," a doctor at one of the hospitals told ITV News.
"From this morning, this early morning, continuous shelling, airstrikes is going on. So civilians in eastern Ghouta are frightened. It’s very hard life here, its a miserable life. We want the world to demonstrate credibility about human rights. We want the the world to stop this crime against civilians in eastern Ghouta."
The government bombardment - backed by Russian forces - has involved warplanes, helicopter gunships and missiles as well as artillery in a major escalation of violence near Assad's seat of power.
The Syrian Observatory for Human Rights said it was the deadliest days in eastern Ghouta since 2015.
The opposition-affiliated Syrian Civil Defence, also known as White Helmets, said many victims are trapped under the rubble.
Rebels retaliated by hitting some Damascus neighbourhoods with mortar shells, killing eight, including three children, and wounding 15 people, according to the state news agency SANA. On Tuesday morning, Damascus residents reported shelling of areas in the centre.
The UN envoy for Syria, Staffan de Mistura expressed concerns that eastern Ghouta could fall victim to widespread bloodshed like that in northern Aleppo more than a year ago.
The UN children's agency, Unicef, issued a one-page statement of protest against the killings and carried a headline, saying: "Do those inflicting the suffering still have words to justify their barbaric acts?"
The International Committee of the Red Cross also issued a statement saying that "this cannot go on". It added that there had been "distressing reports" of dozens injured and killed every day in eastern Ghouta, with "families trapped, with no safe place to hide from shelling. Dozens of mortars in Damascus cause civilian casualties and spread fear. We cannot let history repeat itself."
It said that medical personnel are unable to cope with the high number of wounded and the area doesn't have enough medicines and supplies.
Marianne Gasser, ICRC's head of delegation in Syria, says the "wounded victims are dying only because they cannot be treated in time."
She called for ICRC teams to be allowed inside the area to aid the wounded.