"We were first besieged in an enormous prison called eastern Ghouta, now we are trapped in shelters similar to tombs", Mohsen said.
Nearly 600 people are believed to have been killed and over 2,000 injured, since Syrian government forces launched an air and ground offensive on the rebel-held enclave on February 18, according to the statement.
Near the Syrian capital, Damascus, a five-hour daily truce went into effect Friday for the fourth day this week, but no civilians left the city's rebel-held suburbs known as eastern Ghouta.
Hamza Beriqdar, spokesman of the main rebel faction in Ghouta called the Army of Islam, said in an audio recording shared on social media that the opposition fighters had to retreat from some areas in eastern Ghouta amid a hail of fire from artillery shelling, airstrikes and helicopter attacks, also calling it a "scorched earth policy".
Government forces need to advance just a few more kilometres (miles) further to split the enclave in two, said a commander in the military alliance that backs Assad.
While the relentless bombing has somewhat subsided in eastern Ghouta, home to about 400,000 civilians, the Syrian government's push to squeeze the insurgents out of the region continued.
In a Sunday statement, the White House again condemned "the ongoing military offensive that the Assad regime, backed by Russian Federation and Iran, is perpetrating against the people of Eastern Ghouta".
Zeid Ra'ad al-Hussein, the Head of the United Nations human rights Council says the attacking of the Syrian civilian is a War Crime.
Eastern Ghouta Civil Defence attempted to put out a fire
The U.S. State Department has called the Russian plan a "joke" and the White House on Sunday accused Russia of killing Syrian civilians. Syrian President Bashar said on Sunday that Syrian forces would continue the operation against terrorists in eastern Ghouta "in parallel with opening the way for civilians to leave".
Moscow is now involved in a military operation to defeat Jaysh al-Islam terrorists holed up in eastern Ghouta.
Rebel forces responded Sunday by launching a counteroffensive.
After advances in recent days that saw the regime seize control of 10 per cent of Eastern Ghouta, rebel fighters clashed with regime forces on Sunday in the eastern part of the enclave, the Observatory said.
French President Emmanuel Macron and UN Secretary General Antonio Guterres expressed "grave concern" about the humanitarian situation in the area in a telephone conversation late Saturday.
"Instead of a much-needed reprieve, we continue to see more fighting, more death, and more disturbing reports of hunger and hospitals being bombed", he said in a statement. "This collective punishment of civilians is simply unacceptable", he added in a statement.
Aid convoys have also not been able to deliver supplies to the besieged territory.
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