NEW YORK — Two-thirds of the United Nations Security Council — including the United States, Britain and France — asked Secretary-General Antonio Guterres on Tuesday to investigate attacks on UN-supported medical facilities in northwest Syria, diplomats said.

Syrian President Bashar Assad's forces, backed by Russia, began an offensive on the last major insurgent stronghold three months ago that the United Nations says has killed at least 450 civilians and displaced more than 440,000 people.

The Security Council has been deadlocked on Syria with Russia and China — two of the body's five veto powers — shielding Assad's government from any action during eight years of war.

Britain, France, the United States, Germany, Belgium, Peru, Poland, Kuwait, Dominican Republic and Indonesia delivered a demarche — a formal diplomatic petition — to Guterres over the lack of an inquiry into attacks on UN-supported facilities.

"At least fourteen UN-supported facilities on the list of deconflicted facilities have been damaged or destroyed in northwest Syria since the end of April," they told Guterres, according to the agreed request seen by Reuters.

"We therefore respectfully request that you consider launching an internal UN investigation into attacks that have damaged or destroyed UN-supported facilities in northwest Syria and report back promptly," they said.

They noted that in 2016 former UN Secretary-General Ban Ki-moon had used his discretionary power to open an inquiry into an attack on a Syrian Arab Red Crescent humanitarian convoy in Aleppo.

Guterres' spokesman did not immediately respond to a request for comment on the demarche.

While the United Nations has shared the locations of humanitarian facilities with the parties to the conflict, UN aid chief Mark Lowcock has told the Security Council that dozens of healthcare facilities have been struck since April.

"Is that information used as it's intended, to protect facilities ... or is it being used to target facilities?" Lowcock said to reporters on Tuesday after briefing the council for the seventh time since the Syrian government offensive began.

The 10 members of the Security Council also called on Guterres to investigate why the so-called deconfliction mechanism had failed to deter attacks.

Russia and Syria have said their forces are not targeting civilians or civilian infrastructure and questioned the sources used by the United Nations to verify attacks on health centers.

In a July 16 letter to Guterres and the Security Council, Syrian UN Ambassador Bashar Ja'afari said some 119 hospital and health care centers "have been out of commission since being taken over by terrorist groups" and "no longer serve their original purpose and cannot be considered hospitals, health-care centers or even 'civilian objects' under humanitarian law."

An array of insurgents have a foothold in northwestern Syria. The most powerful is the jihadist Tahrir Al-Sham, the latest incarnation of the former Nusra Front which was part of Al-Qaeda until 2016.

British UN Ambassador Karen Pierce told the Security Council on Tuesday that Ja'afari's letter was an admission of Syrian government attacks on hospitals.

"That is a war crime and it deserves the utmost, deep investigation so that those units responsible, those military commanders responsible, and the politicians who give them their instructions, can be brought to justice," she said. — Reuters