(CNN) UN Security Council members may vote Thursday on a temporary ceasefire for Syria's besieged Eastern Ghouta region, a day after the United Nations' chief deplored the plight of the civilians trapped there as "hell on earth."
A draft resolution put forward by Sweden and Kuwait on Wednesday calls for a 30-day halt in the fighting in the rebel-held Damascus suburb, where intense shelling by Russian-backed Syrian regime forces has taken a heavy toll this week.
If agreed, the ceasefire would allow for the delivery of critical supplies and the evacuation of the wounded.
The Syrian Observatory for Human Rights said Thursday that 335 people had been killed in Eastern Ghouta since Sunday evening, with 79 children and 50 women among the dead. The bombardment has injured another 1,745 civilians, the UK-based monitoring group said.
In response to the regime bombardment, rebel groups have fired into the Syrian capital this week, killing at least 13 people and injuring dozens, state media has reported.
Russian Foreign Minister Sergey Lavrov said Thursday that his country was ready to consider a UN resolution on a ceasefire -- but with some changes.
"We offered very clear language that would state that the ceasefire will not in any way extend to the terrorist groups ISIS and Jabhat al-Nusra, and the factions that cooperate with them and which are systematically shelling residential areas of Damascus," Lavrov said, speaking alongside his Serbian counterpart in Belgrade.
As one of the five permanent members of the Security Council, Russia has the power to veto any draft resolution.
Syria says it is targeting terrorist groups in Eastern Ghouta. But the United Nations and others accuse it of denying civilians their basic rights.
"I am deeply saddened by the terrible suffering of the civilian population in Eastern Ghouta: 400,000 people who live in hell on earth," UN Secretary General Antonio Guterres said Wednesday.
"My appeal for 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, allowing for the evacuation of an estimated 700 people that need urgent treatment that cannot be provided there, and creating also the possibility for other civilians to be effectively treated in the site," he 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 happening in this horrendous way."
Twenty medical facilities have been targeted by regime strikes since Monday, according to the Syrian American Medical Society, adding to the suffering of a civilian population already desperately short on food, water and drugs after years under siege.
The regime onslaught has prompted warnings that the regime of Syrian President Bashar al-Assad, which is backed by the Russian military, is preparing to crush the rebel-held enclave.
The UN Security Council's deliberations come as international outcry over the plight of civilians in Eastern Ghouta, once known as a green oasis outside Damascus, gathers force.
German Chancellor Angela Merkel on Thursday said that European nations -- and Syria's backers -- must do more to halt the violence.
"What we currently see, the dreadful events in Syria, a regime fighting not against terrorists, but against its own people, the killing of children, the destruction of hospitals, all this is a massacre which needs to be condemned. We clearly say no, yet it calls on us to try and play a bigger role so that we can end such a massacre," she said.
"This call also goes for the allies of the Assad regime, especially Iran and Russia. There is a responsibility there."
German Foreign Minister Sigmar Gabriel has spoken to the head of the International Committee of the Red Cross and will speak later with Russia's Lavrov, Merkel said.
In a statement late Wednesday condemning the Eastern Ghouta attacks, the White House singled out Russian and Iranian support for the Syrian regime. "Assad and his deplorable regime must stop committing additional atrocities and must not be further abetted by backers in Moscow and Tehran," it said.
US ambassador to the United Nations Nikki Haley earlier called for "immediate action" to try to save the lives of trapped civilians in Eastern Ghouta.
"It is simply preposterous to claim that these attacks on civilians have anything to do with fighting terrorism," Haley said. "The Security Council must move to adopt a resolution establishing a ceasefire. The United States will support it, as should every member of the Council. As the Secretary-General warned us all, 'eastern Ghouta cannot wait.'"
UK Prime Minister Theresa May also called Wednesday for Syria and its backers, including Russia, to ensure that the violence stops and help is allowed in.
French President Emmanuel Macron, meanwhile, accused the Assad regime of using the fight against terrorism as a "pretext" to target civilians and its opponents and called for a humanitarian truce to allow the evacuation of injured civilians as soon as possible.
As pressure has mounted, the Kremlin has sought to lay the blame for the crisis at the door of the rebel groups in Eastern Ghouta.
Kremlin spokesman Dmitry Peskov said Thursday on a regularly-held call with journalists that those who support terrorists were "responsible for the situation in Eastern Ghouta," Russian state news agency Tass reported.
He added that neither Russia, Syria nor Iran is one of those states, Tass said, "as they are the ones leading the uncompromising on-the-ground battle against terrorists in Syria."
The Russian military conceded Wednesday that conditions in Eastern Ghouta constituted a "critical humanitarian" situation but claimed talks to resolve the conflict had been "derailed."
Moscow said "illegal armed formations" had ignored calls "to stop fighting and lay down their arms." It claimed they were preventing civilians from leaving the conflict zone.
Peskov dismissed claims that Russia was partly to blame for the wave of civilian deaths this week as "unsubstantiated accusations."