Frame grab taken from footage shows air strikes carried out by the Russian air force between Hama and Aleppo.

A “significant increase” of air strikes on Syrian hospitals recently has killed at least 35 patients and medical staff and wounded 72, Doctors Without Borders (MSF) said on Thursday.

The escalation of attacks by air forces the group did not identify, which began in late September, had targeted 12 hospitals in Idlib, Aleppo and Hama provinces, including six supported by MSF, a statement said.

“Overall, six hospitals were forced to close… and four ambulances destroyed,” said the international medical aid group.

“One hospital has since reopened, yet access to emergency, maternity, pediatric and primary health care services remains severely disrupted.”

The statement said “tens of thousands of people have been forced to flee their homes” as a result of the attacks.

“After more than four years of war, I remain flabbergasted at how international humanitarian law can be so easily flouted by all parties to this conflict,” said Sylvain Groulx, MSF chief for Syria.

Russia launched air strikes in Syria on September 30 in support of long-time ally President Bashar al-Assad, whose own forces have been bombing his opponents for years.

And a U.S.-led coalition has also been carrying out an aerial military campaign in the country for more than a year.

Both say they are targeting the extremist ISIS group that has seized large chunks of Iraq and Syria.

More than 250,000 people have died in the Syrian war, now in its fifth year.

Some 6.5 million more are displaced inside the country and another 4.2 million have fled abroad in one of the largest displacement crisis of modern times.

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