More than 400 killed in Syrian airstrikes on rebel areas, UN calls for ceasefire Tuesday was the deadliest day in Syria in nearly two months, activists say.

 LONDON -- Intense airstrikes pounded a rebel-held area just outside Damascus for the third day in a row on Wednesday, despite a call for a ceasefire by the United Nations.

Syrian warplanes killed at least 34 civilians, including 12 children, in Eastern Ghouta on Wednesday, according to the Syrian Observatory for Human Rights, a U.K.-based monitoring group. That brings the death toll for Eastern Ghouta since Monday to 145 civilians, the group said.

The U.N. mission in Damascus warned Tuesday of "dire consequences" of the humanitarian crises in Syria, highlighting seven areas in need of urgent aid. The U.N. officials called for "an immediate cessation of hostilities lasting for at least one month throughout Syria to enable the delivery of humanitarian aid and services, evacuation of the critically sick and wounded, and alleviation of people’s suffering, to the extent possible, wherever they are."

The local government said on Wednesday that all schools were closed due to the heavy bombardments on Eastern Ghouta, home to an estimated 400,000 people who are living with little access to food, fuel and healthcare under the tightening siege.

Syrian and Russian airstrikes and shelling killed 80 civilians, including 19 children, according to the Syrian Observatory for Human Rights, which said it was the deadliest day in Syria in nearly two months.

Images showed rescuers pulling survivors, covered in white dust, from under collapsed buildings. Others depicted dead bodies lined up next to each other on hospital floors. One grieving father was captured on camera screaming as he embraced the blood-soaked body of his young child.

"The Syrian government hit all neighborhoods of Ghouta with all kinds of weapons," Siraj Mahmoud, head of media for the White Helmets in rebel-held rural Damascus, who works under a pseudonym, told ABC News on Tuesday. "The situation is miserable."

A White Helmet rescue worker was on his way to an area that has just been attacked only to realize that his own building was the one hit. He later appeared in a cloud of dust carrying his 3-month-old son, Ibrahim, who he had just rescued from under the rubble.

Amani Ballour, a pediatrician in Eastern Ghouta, said her hospital received more than 100 injured, 40 of them children, on Tuesday. She said the children had various injuries -- from cuts to broken bones.

Of 10 different towns that were hit on Tuesday, the city of Douma saw the fiercest attacks, according to local activists and the Syrian Observatory for Human Rights.

"They are still pulling out victims from under the rubble," Samira, a 45-year-old resident of Douma and aid worker, told ABC News early Wednesday. She asked ABC News not to use her real name out of fear of repercussions. "A five-story building collapsed and it was filled with residents."

Samira sent ABC News a photo of a piece of shrapnel that she said landed in her garden Wednesday as airstrikes continued to pound her neighborhood. Her neighbors from the upper floors were at her apartment seeking shelter from the bombardment.

"We don’t have a shelter in the neighborhood," said Samira, adding that she could hear the sound of strikes on her neighborhood.

"When I hear the sound of the rocket I know that it's not about to hit us. We don't hear the bomb or rocket when it falls on us. We just find our house destroyed over us," she said adding that her house was bombed while she was home last year and that she didn't hear the sound before it happened.

The region has seen an upsurge in violence since late December as Syrian President Bashar al-Assad, with support from Russia and Iran-backed militants, has carried out military campaigns against the last major rebel-held territories in Syria. The Syrian Observatory for Human Rights said the recent escalation has killed more than 400 civilians, including more than 100 children, in Eastern Ghouta.

“Every day I see people who lose their children and children who lose their parents,” Nour Adam, a media activist in Eastern Ghouta who works under a pseudonym, told ABC News.

Syrian government forces and their allies have also escalated attacks against northwestern Syria’s Idlib province, the largest remaining rebel-held area in the country. The offensive against Idlib hit hospitals over the weekend while a suspected chlorine gas attack on the town of Saraqeb caused 11 people to struggle to breathe, according to medics. The Idlib campaign intensified after Syrian rebels shot down a Russian warplane and killed its pilot on Saturday.

U.N. warcrime investigators said in a statement they are probing multiple reports that bombs containing weaponized chlorine have been used against civilians in the two rebel-held towns of Saraqeb, in Idlib, and Douma in Eastern Ghouta. They condemned what they described as "indiscriminate bombardment and deliberate starvation of the civilian population” in Eastern Ghouta, which hasn't received any aid since November.