BEIRUT: 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 Thursday.

The group said the escalating attacks began in late September, though it did not identify who was behind them, and that 12 hospitals had been hit in Idlib, Aleppo and Hama provinces, including six supported by MSF.

Russia began an aerial campaign in support of Syria's government on Sept. 30 and has been accused by several other medical groups of hitting hospitals and field clinics since then.

In total, MSF said, "six hospitals were forced to close... and four ambulances destroyed."

"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.

In addition to Russia's air strikes, Syria's air force and a U.S.-led coalition targeting the ISIS extremist group are also carrying out raids in Syria.

In recent days two medical organisations have directly accused Russia of strikes that have hit their medical facilities.

Last week, the Syrian-American Medical Society (SAMS), said an estimated nine Russian air strikes had hit hospitals or field clinics in Syria, including several of its facilities.

Among them was a field clinic in the town of Sarmin in the northwestern province of Idlib, which was hit on Oct. 20, killing 13 people, including a physiotherapist and a nurse, according to SAMS.

And this week, the Union of Medical Care and Relief Organisations NGO said three hospitals it supports had been targeted in air strikes.

One of the facilities, in Latamneh in Hama province had been targeted by Russian strikes on two occasions, killing ten patients and injuring civilians and medical workers, the confederation of medical organisations said.

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

More than seven million people have been displaced inside the country and another 4.2 million have fled abroad in one of the largest displacement crises of modern times.