Mr.de Mistura called on both parties to the conflict to stop shelling each other's areas and to allow humanitarian convoys to reach east Ghouta, stressing that the priority needs to be stopping the suffering and the tragedy of the civilians there and elsewhere.
600 people have been reported killed in Ghouta since February 18, according to war monitors.
The Russian daily "pause" falls far short of a 30-day ceasefire voted for by the United Nations Security Council on Saturday and yet to be implemented.
For its part, the opposition "Al-Dorar al-Shamia" network said regime forces were trying to infiltrate Eastern Ghouta using rockets that contain the internationally banned incendiary material, Napalm.
The UN held a second emergency meeting on the situation on Friday to debate the failure of a UN ceasefire mandated on Saturday, as well as orders from Syrian ally Vladimir Putin for daily pauses in fighting to allow aid deliveries and medical evacuations.
Turkish troops and Turkey-backed opposition fighters have been fighting against Syrian Kurdish fighters in Afrin since January 20.
Almost 600 people have died and more than 1,000 have been wounded since the Syrian military launched an operation to retake eastern Ghouta from the rebels more than two weeks ago.
He said regime forces took advantage of the "intensive bombardment and the clashes on the edge of Eastern Ghouta to make a limited advance in the Hosh al-Zawahira and Shaifuniyeh districts".
Russian Federation and Turkey said they were able to arrange the evacuation of some 700 civilians from Eastern Ghouta, but outside observers said they could not confirm the evacuation took place, and the Turkish government conceded the civilians were not brought to Turkey for medical treatment as President Recep Tayyip Erdogan wanted.
We express the greatest concern for the situation in Eastern Ghouta, as it continues to deteriorate significantly.
The Observatory said a child suffocated to death and 13 other people fell ill from a suspected chlorine attack on Sunday in the besieged enclave.
"Medical and other essential services are absent and access for humanitarian workers to the city remains precariously limited because the conditions are so risky", Lowcock said.
Trump also spoke late Thursday with German Chancellor Angela Merkel about Syria and Russia's newly announced weapons systems.
Western diplomats said that the Syrian government had not given the final clearances needed for safe access. She said the USA blamed Russian Federation for continuing to train and equip the Syrian military.
The draft text calls for the creation of a new United Nations body that would assign blame for chemical weapons attacks in Syria.
The offensive has heightened tensions between Turkey and its North Atlantic Treaty Organisation ally, the United States, which backs the YPG fighting against Islamic State militants in eastern Syria.