WASHINGTON 

The U.S. voiced "grave concern" Thursday over a series of attacks in northern Syria's Idlib province blamed on the Assad regime and Russia.

"Despite Russia’s claims to be targeting terrorists, these operations have caused dozens of civilian casualties and have targeted first responders as they attempt to save lives on the ground," State Department spokesman Robert Palladino said in a statement.

"These abhorrent attacks on civilian infrastructure and on settlements for internally displaced people must end now," Palladino added.

Along with regime artillery barrages, Russian SU-34 warplanes struck central Idlib and its rural outskirts, leaving 15 dead, including eight children, the White Helmets civil defense group said Thursday.

Regime attacks in Idlib’s de-escalation zone, in which acts of aggression are expressly prohibited, are believed to have killed at least 138 civilians since the start of the year.

Last September, Ankara and Moscow agreed to turn Idlib into a demilitarized zone following a meeting in Russia's coastal city of Sochi between Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin.

The U.S. warned that the latest spate of attacks risks destabilizing the surrounding region, thereby furthering Syria's humanitarian catastrophe.

"As a party to the September 2018 Idlib ceasefire agreement with Turkey, Russia bears full responsibility for these offensive operations against northern Hama and southern Idlib," Palladino said.

"We call on all parties, including Russia and the Syrian regime, to abide by their commitments and de-escalate violence in the area, protect all civilians, including humanitarian aid workers, and allow unhindered humanitarian access to address the humanitarian disaster created by the Assad regime’s forces," he added.