Rescuers look for survivors at a site damaged after airstrikes on the Syrian rebel-held city of Idlib late Monday. (Khalil Ashawi/Reuters)

Intense airstrikes overnight Tuesday in rebel-held areas of northern Syria killed or wounded scores of people, most of them civilians, as an already fraying peace process wobbled on the verge of collapse.

Aid agencies and human rights monitors said as many as 50 people died when warplanes repeatedly struck buildings around the National Hospital in the capital of Idlib province. They said Russian warplanes were responsible, although that could not be independently confirmed.

The attack on the capital, also called Idlib, followed hundreds of airstrikes over the weekend in and around the city of Aleppo. Activists said those attacks also were carried out by Russian warplanes, as well as Syrian government aircraft.

The strikes are presenting a major challenge to the three-month-old cessation of hostilities in Syria negotiated by Russia and the United States, a truce that was reimposed last month after a previous near-collapse. The partial cease-fire was aimed primarily at reducing attacks on civilian areas, creating space for the delivery of humanitarian aid to besieged communities and building trust at peace negotiations in Geneva.

A Syrian child is pulled out of the rubble of a building said to have been targeted by a Russian airstrike in the rebel-held city of Idlib. (Reuters)

But little of the intended aid has made it through, the peace process has stalled, the opposition’s chief negotiator has resigned, and airstrikes against civilian areas appear to be intensifying again.

“Nothing significant has been achieved even on the humanitarian level,” said Emile Hokayem of the International Institute for Strategic Studies. “The problem is that the current diplomacy has revealed the limits of how far this can go.”

According to aid agencies and monitors, as many as 50 people died and more than 250 were injured when three warplanes repeatedly struck residential buildings clustered around the National Hospital in Idlib for an hour starting about 10 p.m. Monday. The Britain-based Syrian Observatory for Human Rights put the toll at 23 but said it could rise because some people were still buried in the rubble.

The clustering of the attacks suggested that the hospital may have been a target, but it was not directly hit, said Hakan Bilgin, a spokesman for the French aid agency Doctors of the World, which supports the hospital with funding and supplies. Another hospital supported by the charity, in the province of Aleppo, also was damaged in one of the strikes, but there were no casualties, Bilgin said.

Russia did not comment on the strikes, but it had warned Friday that unless the United States responded positively to Moscow’s long-standing request to conduct joint airstrikes in Syria with the Russian air force, the fighting would escalate.

Idlib is located in an area controlled by an alliance of rebel groups that includes the al-Qaeda-linked Jabhat al-Nusra, a designated terrorist organization that is not covered by the cease-fire. However, the terms of the cessation-of-hostilities agreement specify that attacks on civilian targets such as hospitals and residential neighborhoods should stop.

Rescuers carry an injured woman on a stretcher at a site damaged after airstrikes on Idlib, Syria, late Monday. (Khalil Ashawi/Reuters)

The violence, in any case, has been gradually ticking up. Pro-government forces have recaptured areas of the eastern Damascus suburbs that have been under rebel control for nearly four years, and they have been pounding other rebel-held areas around the capital with airstrikes and barrel bombs.

The United Nations had set a June 1 deadline for the delivery of humanitarian aid to an estimated 1 million people, some of them on the verge of starvation in communities surrounded by government forces. But after months of negotiations with the government to secure access to the communities and with the deadline approaching, only 160,000 people have been reached, the United Nations said Monday.

There is still no date for the resumption of stalled peace talks in Geneva, and U.N. special envoy Staffan de Mistura has said he will not reconvene the talks until more progress is made on issues such as the delivery of aid and the suspension of airstrikes.