The International Committee of the Red Cross has called for immediate humanitarian access
The United Nations has called for an end to the "hell on earth" violence in Syria after hundreds of civilians were killed in eastern Ghouta.
Residents in the rebel-held area outside Damascus say they are waiting for their "turn to die" after more than 290 people including dozens of children were killed since Sunday.
UN secretary general Antonio Guterres said the Syrian government's bombing campaign had turned the region into "hell on earth" for civilians.
"My appeal to all those involved is for an immediate suspension of all war activities in eastern Ghouta allowing for humanitarian aid to reach all those in need," he said.
French president Emmanuel Macron has called for a humanitarian truce to allow civilians to be evacuated.
The International Committee of the Red Cross has called for immediate humanitarian access to the rebel-held district.
Rockets and barrel bombs continued to fall on Wednesday - killing at least 24 people - in an apparent preparation for a government ground assault.
The UN has described the situation as "beyond imagination," while Amnesty International said "flagrant war crimes" were being committed.
Bilal Abu Salah, who lives in the eastern Ghouta town of Douma, said: "We are waiting our turn to die. This is the only thing I can say."
International leaders have called on President Bashar al Assad's regime and its ally Russia "to ensure this violence stops and those people in need of help are given that help."
But the Kremlin said claims the Russian military was responsible for civilian casualties in eastern Ghouta were "unfounded."
"I am deeply saddened by the terrible suffering of the civilian population in eastern Ghouta: 400,000 people who live in hell on earth."— United Nations (@UN) February 21, 2018
-- @antonioguterres on the situation in Syria. https://t.co/aMQXUlAQ8V pic.twitter.com/0fjHShLDkQ
An Amnesty International spokesman said: "The Syrian government, with the backing of Russia, is intentionally targeting its own people in eastern Ghouta.
"People have not only been suffering a cruel siege for the past six years, they are now trapped in a daily barrage of attacks that are deliberately killing and maiming them, and that constitute flagrant war crimes."
The Syrian government maintains it is fighting a war on terrorism and does not target civilians.
State media reported that rebels have been firing mortars on districts of Damascus near eastern Ghouta, killing at least six people on Tuesday and wounding two people on Wednesday.
A spokesman for Russia's Defence Ministry said there had been a "massive bombardment by illegal armed groups from eastern Ghouta" which had targeted residential areas, hotels and Russia's Centre for Syrian Reconciliation.
The UN has called for a ceasefire, saying the situation for civilians in eastern Ghouta is "spiralling out of control."
It has warned the violence could turn into a repeat of the battle for Aleppo, which endured months of conflict between rebels and government forces in 2016.
Eastern Ghouta was among the first Syrian regions to shake off government rule after popular demonstrations against President Assad swept through the country in 2011, eventually leading to civil war.
It is also among the last places to resist Assad's determined campaign to take back control of every last rebel-held region.
It is supposed to be one of the "de-escalation zones" agreed by Russia, Iran and Turkey as part of their diplomatic efforts. But a former al Qaeda affiliate, which has a small presence there, is not included in the agreement.
End attacks. Lift the siege.— AmnestyInternational (@amnesty) February 21, 2018
End attacks. Lift the siege.
End attacks. Lift the siege.
Call on Russia & Syria to stop killing civilians in #EasternGhouta IMMEDIATELY → https://t.co/qV1QBMeWCg pic.twitter.com/27karukyQL
The bombardment of eastern Ghouta by Assad's forces resulted in the worst 48-hour death toll in Syria since a chemical attack in 2013.
The Syrian Observatory for Human Rights said 106 civilians, including 19 children, were killed in the violence on Tuesday.
It came after 127 people were killed on Monday in eastern Ghouta's bloodiest day in four years.
The Observatory blamed Russian warplanes, saying Moscow carried out its first strikes in three months on eastern Ghouta.
In another development, Assad's forces were sent to the northern Afrin region, where they came under fire by Turkish forces attacking the Kurdish-controlled area.
Syria is sending in forces to come to the aid of a Kurdish militia known as the YPG, after Turkey and its Free Syrian Army allies made unexpected gains there.
A spokesman for Turkey's Prime Minister Recep Tayyip Erdogan said there would be "serious consequences" after a convoy of about 50 vehicles tried to enter Afrin on Tuesday but were repelled by artillery fire.