Ongoing attacks by Syrian government forces and their allies has created "hell on Earth" for civilians stuck in a rebel-held Damascus suburb, the UN's chief said on Monday, as air strikes and ground operations continued despite a days-old ceasefire.
Russia's President Vladimir Putin, meanwhile, ordered a "humanitarian pause" from 9am-2pm local time on Tuesday to allow civilians to evacuate Eastern Ghouta.
The bombardment of rebel-held enclave over the past week has been one of the heaviest of Syria’s seven-year war, killing more than 550 people in eight days, according to a toll compiled by the Syrian Observatory for Human Rights, a UK-based war monitor.
Rebel shelling has caused 36 deaths and a number of injuries in Damascus and nearby rural areas in the last four days, Zaher Hajjo, a government health official, told Reuters news agency.
United Nations Secretary-General Antonio Guterres called for an immediate implementation of Saturday's Security Council resolution for a 30-day ceasefire in Syria.
Speaking at the UN Human Rights Council in Geneva, Switzerland, Guterres described the situation in Eastern Ghouta as "hell on Earth".
"I remind all parties of their absolute obligation and international humanitarian and human rights law to protect civilians and civilian infrastructure at all times," he said.
"Similarly, efforts to combat 'terrorism' do not supersede these obligations," he added.
The remarks come as doctors in the enclave accused the Syrian government of launching a chlorine gas attack in the town of Al-Shifaniyah in Eastern Ghouta.
Syria's Civil Defence rescue team, also known as the White Helmets, said on Sunday at least one child died as a result of suffocation.
According to the Syrian opposition's health officials, victims were showing symptoms "consistent with exposure to toxic chlorine gas".
Russia's foreign minister denounced the allegations that gas was used.
"There are already bogus stories in the media that yesterday chlorine was used in Eastern Ghouta, citing an anonymous individual living in the United States," Sergey Lavrov said at a press conference.
On Sunday, President Bashar al-Assad's forces launched a ground offensive against opposition groups from multiple fronts, in an attempt to penetrate into the besieged enclave, which has been under rebel control since 2013.
The two major rebel factions in Eastern Ghouta are Jaish al-Islam and Failaq al-Rahman. Tahrir al-Sham, an alliance of opposition fighters, also has a small presence there.
Al Jazeera's Zeina Khodr, reporting from Beirut in neighbouring Lebanon, said the government has not been able to take "any inch of territory in Ghouta" since the ground offensive began.
At least 16 people have been killed since Monday morning in Eastern Ghouta's Douma, local activist Alaa al-Ahmed told Al Jazeera from the enclave.
A day earlier, at least 27 people in the Damascus suburb died as a result of shelling by Russian-backed Syrian warplanes.
Last week, deadly air strikes and artillery fire launched by Syrian forces and their allies exacerbated a dire humanitarian crisis in the besieged enclave, home to some 400,000 people.
Fighting is raging elsewhere in Syria as Turkey presses its offensive against a Kurdish militia in Afrin, rival rebel groups fight each other in western Idlib province, and a US-led coalition targets Islamic State of Iraq and the Levant in the east.