United Nations Secretary-General Antonio Guterres strongly condemned on Thursday airstrikes against hospitals in northwest Syria, saying civilian and civilian infrastructure must be protected. He voiced his concern after reports that four health facilities were hit during a single day of bombing. An ambulance center, a clinic and two hospitals including one in Maarat al Numan that is among the largest in the area were attacked on Wednesday, according to the Union of Medical Care and Relief Organizations, a Syrian doctors group. "Civilians and civilian infrastructure, including medical facilities, must be protected," said Guterres in a statement released by his spokesman. The United Nations had shared the coordinates of the hospital in Maarat al Numan with the warring parties, the statement said. "Parties to the conflict must respect their obligations under international humanitarian law," Guterres said, adding that those behind the attacks must be held accountable. Backed by Russia, Syrian regime forces have since late April ramped up bombardment of the Idlib region, which is controlled by the extremist group Hayat Tahrir al-Sham (HTS). The United Nations has warned that an all-out offensive to push out the opposition could lead to a bloodbath in the region where some three million people live. UN aid chief Mark Lowcock last month said he had asked Russia to explain how it uses data on the location of Syrian hospitals following a string of attacks on health facilities. More than 23 hospitals have been hit by strikes since late April in a campaign that Western powers have said is aimed at sowing terror among civilians. Russia has firmly denied that the bombing campaign has targeted hospitals and maintains the military operation is aimed at driving out "terrorists." At least 544 civilians have been killed and over 2,000 people injured since the Russian-led assault began two months ago, rights groups and rescuers said on Saturday. Residents and rescuers say the two-month-old campaign has left dozens of villages and towns in ruins. According to the United Nations, at least 300,000 people have been forced to leave their homes for the safety of areas closer to the border with Turkey. The war in Syria, now in its ninth year, has killed more than 370,000 people and displaced millions since it started with the brutal repression of anti-regime protests.