Nikki Haley, the US ambassador to the UN, called on the Security Council to pass a draft resolution calling for a 30-day ceasefire in Eastern Ghouta, a besieged rebel enclave in Syria.
"It’s time to take immediate action in the hopes of saving the lives of the men, women, and children who are under attack by the barbaric Assad regime," Haley said in a statement on Wednesday.
The US diplomat called for the ceasefire to allow aid to be delivered to Ghouta.
Top UN officials also demanded an immediate cessation of aerial attacks on Syria's devastated Eastern Ghouta with Secretary-General Antonio Guterres calling the rebel-held enclave "hell on Earth".
Syrian forces backed by Russian warplanes continued to pound the Damascus suburb leaving at least 27 killed on Wednesday, bringing the civilian death toll to more than 270 - including 60 children - over the past three days.
"My appeal to all those involved is for an immediate suspension of all war activities in Eastern Ghouta," Guterres told a UN Security Council meeting. "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."
About 400,000 people live in the area, which has been besieged for years by regime forces with few supplies reaching the desperate population.
UN human rights chief Zeid Ra'ad al-Hussein called the air attacks and artillery fire a "monstrous campaign of annihilation.
"International humanitarian law was developed precisely to stop this type of situation, where civilians are slaughtered in droves in order to fulfill political or military objectives," he said in a statement.
The UN has documented 346 civilian deaths with 878 wounded in Eastern Ghouta since February 4, mostly from air attacks hitting residential areas, Zeid said.
Russia involvement
Russia denied involvement in air attacks on Wednesday, calling an accusation by a US official that Moscow was taking part in the raids "groundless".
"It is not clear what they are based on ... We do not agree," Russian presidential spokesman Dmitry Peskov said.
Russian jets have carried out air attacks in Syria in support of President Bashar al-Assad since 2015 - greatly changing the course of the seven-year war in the regime's favour.
Negotiations to try to peacefully resolve the dire situation in Eastern Ghouta broke down after rebels there ignored calls to cease their resistance and lay down their arms, the Russian military said late on Wednesday.
Rebels were preventing civilians from leaving the conflict zone, Russia's ceasefire monitoring centre in Syria, which is run by the Russian military, also said in a statement.
Russian UN Ambassador Vassily Nebenzia called for a UN Security Council meeting on the situation in Eastern Ghouta to be held on Thursday.
"The fighting appears likely to cause much more suffering in the days and weeks ahead," ICRC's head in Syria Marianne Gasser said in a statement.
"Wounded victims are dying only because they cannot be treated in time. In some areas of Ghouta, entire families have no safe place to go."
Eight medical facilities were attacked on Tuesday.
Speaking to Al Jazeera from Eastern Ghouta, activist Mouayad Mohildeen said shelling and bombing raids continued to target the enclave.
"There's a lot of dead people here, there's a lot of injuries. Medical points are out of service. We feel betrayed by the international community," he said.
Reporting from Beirut, Al Jazeera's Zeina Khodr said the ongoing air campaign could be a precursor to a ground invasion.
The regime has sent military reinforcements to the front lines around the besieged suburb.
Pro-regime newspaper Al Watan has said the stepped-up bombing campaign comes in the face of a ground operation that may start any time.
"A ground offensive will not be easy, the regime and its allies have repeatedly tried to storm Eastern Ghouta in the past.
"The rebels have strong defenses and an underground tunnel network that they use to their advantage," Khodr said.
Since March 2011, an estimated 465,000 Syrians have been killed in fighting, one million wounded, and about 12 million - half the country's prewar population - displaced from their homes.
PHOTO CAPTION
FILE PHOTO - A man is seen with a child who rides a bicycle inside damaged area in Misraba, Eastern Ghouta, near Damascus, Syria January 11, 2018. REUTERS
Al-Jazeera