A picture taken on April 30, 2019 shows a medical room covered in rubble and debris following reported shelling by the Syrian government in the Syrian town of Hbeit in the south of Idlib province. (Xinhua/AFP)

UNITED NATIONS, May 6 (Xinhua) -- United Nations Secretary-General Antonio Guterres was alarmed by reports of aerial attacks on population centers in Syria, said a statement issued Monday by his spokesman.

"The secretary-general is following with great concern the intensifying hostilities in the de-escalation area of northwestern Syria involving Syrian government forces and their allies, armed opposition forces, and Hayat Tahrir al-Sham. He is alarmed by reports of aerial attacks on population centers and civilian infrastructure resulting in hundreds of civilian dead and injured and over 150,000 newly displaced persons," the statement said.

On May 5, three health facilities were reportedly hit by airstrikes, bringing the total of such attacked medical venues to at least seven since April 28. Nine schools have reportedly been hit since April 30, and schools in many areas have been closed until further notice.

The secretary-general urged all parties to uphold international humanitarian law and protect civilians.

"He calls for an urgent de-escalation of the situation as the holy month of Ramadan begins and urges the parties to recommit fully to the ceasefire arrangements of the memorandum signed on Sept. 17, 2018."

"He urges the Astana guarantors to ensure that this takes place," the statement added.

The Astana process was launched by Russia, Iran and Turkey in the Kazakh capital of Astana in January 2017 to bring all warring parties in the Syrian conflict to the table to find a political solution as a complementary part of the UN-sponsored peace talks in Geneva.

The most significant outcome was the establishment of four de-escalation zones in Syria.