The World Health Organization on Wednesday said Russia is in violation of international humanitarian law based on several reported attacks on Ukrainian hospitals and health workers.

Several of the reports are unconfirmed, but in at least one confirmed case, a hospital came under a “heavy weapons attack” that killed four and injured 10 others, including six health workers. WHO Director-General Dr. Tedros Adhanom Ghebreyesus said the agency is working to confirm several other reports.

“The sanctity and neutrality of health care—including of health workers, patients, supplies, transport and facilities—and the right to safe access to care, must be respected and protected,” Tedros said in a press briefing Wednesday. “Attacks on health care are in violation of international humanitarian law.”