The U.N. aid chief said Tuesday he had asked Russia to explain how it uses data on the location of Syrian hospitals and clinics following a string of attacks on health facilities.

Mark Lowcock, the undersecretary-general for humanitarian affairs, told the U.N. Security Council he was "not sure" hospitals that share coordinates of their location under a U.N. "deconfliction" system will be protected.

More than 23 hospitals have been hit by strikes since Russia-backed Syrian forces launched an offensive in the opposition-controlled Idlib region in late April, according to the U.N.

On June 20, an ambulance transporting an injured woman in southern Idlib was hit, killing the woman and three medical workers. "I have written to the Russian Federation to request information as to how the details provided through the deconfliction mechanism are used," Lowcock told the council.

Russia, which backs Syrian forces in their offensive in the northwest, has firmly denied that the bombing campaign has targeted hospitals in the Idlib region.

Lowcock also stated that violence in the northwestern Syria intensively continues despite the ongoing efforts for a ceasefire and called actors to stop the tension and save people's lives. Since late April, Idlib has come under relentless bombardment by the Bashar Assad regime and its ally Russia, which so far has killed nearly 500 civilians. The violence has forced around 330,000 people to flee their homes and has damaged 23 health centers, the United Nations said.

As the opposition's last enclave, Idlib's pre-war population of 1.5 million has swelled to around 3 million with new refugee waves after it was designated a "de-escalation zone" under the Astana agreement between Turkey, Russia and Iran in May 2017 to pave the way for a permanent political solution in Syria.

Turkey and Russia inked a deal for a buffer zone in September in Sochi to prevent a massive regime offensive on the Idlib region, near the Turkish border.

Following eight months of calm provided by the Sochi deal, the Bashar Assad regime intensified its attacks starting April 26, under the pretext of fighting Hayat Tahrir al-Sham (HTS) militants holed up in Idlib.

It was also reported that regime forces killed two members of the Syrian Civil Defense while four more were wounded when an airstrike struck their ambulance in northwestern Syria. The Syrian Civil Defense, also known as the White Helmets, said the attack occurred in Khan Sheikhoun, a town which has been repeatedly targeted over past weeks.