(CNN) At least 250 civilians have been killed in two days of relentless bombardment of Eastern Ghouta in Syria, activists say, prompting warnings that the regime of Bashar al-Assad is preparing to crush the rebel-held enclave.

The UK-based Syrian Observatory for Human Rights said 106 died on Tuesday, the highest in a single day since a 2013 alleged chemical attack on eastern Ghouta, which activists say killed approximately 1,400 people.

The intensified bombardment of Eastern Ghouta, an area outside Damascus that has been besieged by the regime of Bashar al-Assad for years, drew international condemnation. Amnesty International said "flagrant war crimes" were being committed on an "epic scale" there, and the UN children's agency UNICEF published a blank statement, saying in a footnote there were "no words" to describe the suffering of children.

"For six years, the international community has stood by as the Syrian government has committed crimes against humanity and war crimes with total impunity," said Amnesty International's Syria researcher, Diana Semaan.

"People have not only been suffering a cruel siege for the past six years, they are now trapped in a daily barrage of attacks that are deliberately killing and maiming them, and that constitute flagrant war crimes."

Injured children are treated at a hospital in rebel-held Douma, Eastern Ghouta, on Monday.

The Syrian Observatory said the death toll included 58 children and 42 women. More than 1,200 people were wounded by constant shelling of the besieged area outside Damascus. The assault continued into Wednesday.

"These are the worst days of our lives in Ghouta," Eastern Ghouta hospital director and pediatrician Amani Ballour said.

"We in Ghouta have been getting hit by airstrikes for more than five years and this is not new to us ... but we have never seen anything like this escalation."

Doctors said medics were working around the clock treating hundreds of injured people. Several medical facilities in Eastern Ghouta were reported to have been struck on Monday.

Medical supplies were already in short supply due to a yearslong siege of the area that began in 2012. Now, Syrian regime forces are accelerating their offensive against the suburb, one of the last rebel-controlled areas in the country.

Various Islamic rebel groups control Eastern Ghouta, including the al Qaeda-linked Hayat Tahrir al-Sham, which was previously known as Jabhat al Nusra before renouncing its ties to al Qaeda.

Photos: Airstrikes in Eastern Ghouta Medics tend to a baby as a child cries next to them at a makeshift clinic in Douma, Syria, on Thursday, February 23. More than 400 civilians have been killed this week in Syria's rebel-held Eastern Ghouta region, according to the head of the region's health department on Friday, February 23. Syrian regime forces have been pounding Eastern Ghouta with shells, mortars and bombs. Hide Caption 1 of 15 Photos: Airstrikes in Eastern Ghouta A man rescues a child after a reported airstrike in the rebel-held town of Hamouria on Wednesday, February 21. Syria says it is targeting terrorists in Eastern Ghouta. Hide Caption 2 of 15 Photos: Airstrikes in Eastern Ghouta Members of a Syrian civil-defense team rescue a man in Hamouria on February 21. Hide Caption 3 of 15 Photos: Airstrikes in Eastern Ghouta A woman and children run for cover after bombing in Hamouria on Monday, February 19. Hide Caption 4 of 15 Photos: Airstrikes in Eastern Ghouta Hide Caption 5 of 15 Photos: Airstrikes in Eastern Ghouta An injured child cries as he receives treatment at a makeshift hospital in Hamouria on February 19. Hide Caption 6 of 15 Photos: Airstrikes in Eastern Ghouta A man carries an injured victim amid the rubble of buildings. Hide Caption 7 of 15 Photos: Airstrikes in Eastern Ghouta A boy waits to receive medical treatment at a field hospital on February 19. Hide Caption 8 of 15 Photos: Airstrikes in Eastern Ghouta Injured children are treated at a hospital in Douma. Hide Caption 9 of 15 Photos: Airstrikes in Eastern Ghouta A wounded boy receives treatment in Douma following airstrikes on the village of Mesraba. Hide Caption 10 of 15 Photos: Airstrikes in Eastern Ghouta A man carries an infant he rescued from the rubble in Hamouria on February 19. Hide Caption 11 of 15 Photos: Airstrikes in Eastern Ghouta Injured children are treated at a hospital in Douma. Hide Caption 12 of 15 Photos: Airstrikes in Eastern Ghouta Ghaith, a wounded 12-year-old boy, cries as he waits for treatment -- and news of his mother in the operating room -- at a makeshift hospital in Kafr Batna. They were hurt in airstrikes on the town of Jisreen. Hide Caption 13 of 15 Photos: Airstrikes in Eastern Ghouta Children cry at a makeshift hospital in Douma. Hide Caption 14 of 15 Photos: Airstrikes in Eastern Ghouta A man weeps over his child, who was killed in the Mesraba airstrikes. Hide Caption 15 of 15

"I can tell you that the situation is very catastrophic ... there were four hospitals that were destroyed and cannot be able to continue their work in helping people in Eastern Ghouta," Dr. Fares Ouraiba said from the Damascus suburb. He said most of the dead were women and children.

Rebels responded by firing into Damascus. Syrian state TV, SANA, said at least 13 people were killed and 77 others were wounded when 114 rockets and mortar rounds landed on several neighborhoods in Damascus.

SANA added that the Syrian army responded to the attacks with "precise strikes," destroying rocket launchers and fortified positions used by the armed groups.

Desperation in Eastern Ghouta

Nearly 400,000 people live in Eastern Ghouta. They account for 94% of all currently besieged Syrians, according to the United Nations, and many are in desperate need of humanitarian aid.

A UN and Syrian Arab Red Crescent aid convoy arrived in Eastern Ghouta last Wednesday. It was the first convoy to enter the area since November, Reuters reported.

Smoke rises from buildings in Mesraba, Eastern Ghouta, after a bombardment on Monday.

Eastern Ghouta is meant to be one of the so-called "de-escalation zones" agreed to in a deal struck by Russia, Turkey and Iran last year. In theory, such zones, also referred to as nonconflict or safe zones, are meant to be areas where civilians can live without being targeted by any party in Syria's war.

Attacks in Eastern Ghouta in recent weeks have provoked an international outcry. Since November, hundreds of civilians have been killed or injured in airstrikes and shelling across the country, according to the United Nations.

UN Secretary-General António Guterres "is deeply alarmed by the escalating situation in Eastern Ghouta and its devastating impact on civilians," according to a statement from a spokesman.

"The United Nations has repeatedly called for a cessation of hostilities to enable humanitarian aid deliveries, and the evacuation of the sick and wounded," the statement said.

Why now?

With ISIS having lost its territorial footing in the past year, Russian-backed Syrian forces are making a major push to overrun the country's remaining rebel strongholds.

The offensive in Eastern Ghouta is developing in parallel to a campaign in the northwest province of Idlib, another so-called safe zone that hosts more than 1 million internally displaced people.

What's next?

Residents of Eastern Ghouta are bracing themselves for what they believe is an imminent ground invasion by Syrian regime forces. They said events in their suburb are playing out similarly to the 2016 offensive in Aleppo, when rebels and ISIS militants were expelled by a government offensive that reduced much of the city to rubble.

"This could be one of the worst attacks in Syrian history, even worse than the siege on Aleppo," Zedoun al Zoebi, CEO of the Paris-based Union of Medical Care & Relief Organizations said in a statement Monday. "The sheer intensity of airstrikes is leveling the city, and killing civilians without any regard or mercy."

What's the world doing?

International monitors are sounding alarm bells about the offensive in Eastern Ghouta, but aside from the aid convoy that made it into the area last week, little else has changed in recent days aside from an uptick in the bloodshed.

Amnesty International accused the world of failing to act. "In the end the Syrian government has followed the systematic policy, the systematic strategy of besiegement and bombardment until people have to give up and then they are forcibly displaced," said Leen Hashem, Amnesty International's regional campaigner on Syria.

"What we have seen in Aleppo we're seeing again in Ghouta today. Not even a proper aid convoy is being allowed to enter Ghouta, and the international community is just watching."

#RunningOutOfWords



Statement from @gcappelaere on the war on children in #Syria



Reports of mass casualties among children in Eastern #Ghouta and Damascus#ChildrenUnderAttack pic.twitter.com/X2FYJ4OPnf — UNICEF MENA (@UNICEFmena) February 20, 2018

UNICEF condemned the "war on children" in Syria. "UNICEF is issuing this blank statement. We no longer have the words to describe children's suffering and our outrage," the agency said in a footnote. "Do those inflicting the suffering still have words to justify their barbaric acts?"