Air strikes in northern Syria have hit at least 12 hospitals in recent weeks, killing at least 35 patients and medical staff in a new escalation of fighting, international medical charity Medecins Sans Frontieres said on Thursday.

The violence has also caused the displacement of thousands of civilians since the end of September, MSF said, reporting a significant increase strikes on medical facilities.

The charity did not specify which country had carried out the air strikes. Russian and Syrian jets have been carrying out an intense air offensive in the west and northwest Syria.

Along with the dozens killed, another 72 were wounded in the strikes on hospitals in Aleppo, Idlib and Hama provinces, and six hospitals were forced to close, MSF said in a statement.

Fighting between rebels and forces loyal to President Bashar al-Assad has intensified since Russia intervened in the war on the side of the government with an air campaign that began on Sept. 30.

In the past month, another 1,700 families have joined 110,000 people already sheltering in camps in Idlib province, MSF said, making it difficult for aid groups to cope with the influx.

Head of MSF for Syria, Sylvain Groulx, condemned the actions of warring parties.

"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," he was quoted as saying in the statement.

(Reporting by John Davison; Editing by Raissa Kasolowsky)