The United Nations on Thursday voiced concern over civilian suffering in northwest Syria as airstrikes and shelling escalate.

"The UN remains gravely concerned over the safety and protection of some 4.1 million women, children and men in northwest Syria, including some 2.1 million internally displaced people, following an intensification of airstrikes and shelling in recent days in Idlib," said Stephane Dujarric, spokesperson for UN Secretary-General Antonio Guterres.

Shelling was reported in 13 communities inside Idlib governorate, killing at least two people and injuring 20. Shelling was also reported in several neighborhoods in west Aleppo city, leaving at least one man dead and six people wounded, said Dujarric, citing Wednesday's reports from the UN Office for the Coordination of Humanitarian Affairs.

An airstrike on Wednesday reportedly impacted the Orient Hospital in the town of Kafr Nabl in Idlib, injuring several people and rendering the hospital inoperable, Dujarric told a regular press briefing.

On Thursday, a large explosion in one of the production units of Syria's Banias refinery, the largest in the war-torn country, left one worker killed and one engineer injured.

More than 1,000 people have lost their lives – many of them children – and over 400,000 have been displaced in Syria's northwest since the end of April, he said.

"The UN continues to remind all parties of their obligations to protect civilians and civilian infrastructure in line with international humanitarian law and international human rights law," said the spokesman.

(With input from Xinhua News Agency and Reuters)