BEIRUT — Bunker-busting bombs are putting children in so much danger in Syria’s Aleppo that they won’t even be able to return to underground schools when classes resume this weekend, a charity said Friday.

The "ferocious assault" on Aleppo could deprive almost 100,000 school-age children of an education, said Save the Children, which supports 13 schools in the northern battleground city, including eight underground.

The United States has accused Russia of using "incendiary bunker-busting bombs" in a week-long aerial assault on the city’s rebel-held east in support of Syria’s President Bashar al-Assad.

The Kremlin said Thursday it would press on with its bombing campaign, blaming the surge in violence on Washington’s failure to control rebels in Aleppo and insisting Syrian forces were battling "terrorists".

Save the Children said the use of the "earthquake bombs" which burrow four to five metres (yards) into the ground before exploding meant even underground schools were unsafe.

"Schools in eastern Aleppo were due to re-open for the new school year tomorrow, but as the city continues to suffer a ferocious assault they will remain closed, depriving almost 100,000 school-age children of an education, while they continue in fear for their lives," it said in a statement.

The bunker-busting bombs are used to penetrate hardened targets such as underground military headquarters, leaving victims entombed in rubble.

"Parents are afraid to send their children to school because everything is targeted," said a school principal in eastern Aleppo only identified as Omar.

"The students are also suffering on all levels, you see them barely walking, dragging themselves, which makes them unable to focus on the learning and studying," Save the Children quoted him as saying.

"Regarding the bunker buster bombs, of course only hearing the sound creates a state of terror and panic that is not like anything else.

"The immense power of destruction is the most important, it can destroy underground shelters and basements and the buildings get totally destroyed, not just partially."

Nowhere is safe

Save the Children said more than 300 children have been killed or wounded in eastern Aleppo in the past five days.

"We are not going to school because the airplanes bomb any gathering," said 12-year-old Amjad.

"When the plane comes we sit on the floor, afraid that things might fall above us. One of my friends died in the bombing -- he was my best friend.

"I love to go to school to study and I wish I could become a civil engineer to rebuild the houses that were destroyed," he said, quoted by the charity.

Even before the latest escalation, school enrolments in Aleppo had fallen to as low as six percent, said Save the Children.

"We’re now more likely to see children being pulled from the rubble or treated on the floor of a hospital than sat at a school desk," said Nick Finney, Save the Children head for northwest Syria.

"Children deserve the right to play, to learn, to be children.

"The use of bunker busting bombs means there is literally nowhere we can keep children safe, and we want to see the use of these weapons investigated as a potential war crime."

Obama, Merkel condemn ’barbarous’ Russian, Syrian strikes on Aleppo

US President Barack Obama and German Chancellor Angela Merkel condemned what they called "barbarous" Russian and Syrian regime airstrikes on Aleppo during a phone call Thursday, the White House said.

The Syrian government and its ally Russia "bear special responsibility for ending the fighting in Syria," the two leaders agreed, strongly condemning the strikes in eastern Aleppo, an area they said is "populated with hundreds of thousands of civilians, half of whom are children."

The United Nations says a humanitarian catastrophe is unfolding unlike any witnessed so far in Syria’s brutal five-year war as Moscow and Damascus carry out an offensive to retake the city after a ceasefire collapsed last week.

Russia and the United States have traded blame for the collapse of the deal that would have marked the first step in a new effort to end the war that has killed 300,000 people since 2011.

Obama and Merkel urged parties on the ground to "continue respecting the ceasefire, begin the disengagement of forces along the line of contact as quickly as possible and provide international monitors unfettered access to the entire conflict zone."

"The president and chancellor strongly condemned the barbarous Russian and Syrian regime airstrikes against eastern Aleppo," the White House said.

Two of the largest hospitals in the city’s east were bombed on Wednesday in what UN Secretary-General Ban Ki-moon described as a war crime.

Air strikes pounded Aleppo province on Thursday while at least 11 civilians, including seven children, died during attacks on the city of Idlib, nearby Jarjanaz and central Hama province, the Syrian Observatory for Human Rights said. — AFP