Attacks on hospitals, ambulances and other health care facilities in Ukraine have increased rapidly in recent days and the country is running short of vital medical supplies, the World Health Organization (WHO) said on Tuesday.

The U.N. agency confirmed on Monday that at least nine people had died in 16 attacks on health care facilities since the start of a Russian invasion of Ukraine on Feb. 24. It did not say who was responsible.

The WHO's senior emergency officer for Europe, Catherine Smallwood, told a news briefing that the tally included incidents where ambulances had been commandeered for purposes other than emergency health care.

"We will continue to update those numbers. They've been increasing quite rapidly over the past few days," Smallwood said.

The agency is working to rapidly provide medical supplies to Ukraine, where oxygen, insulin, personal protective equipment, surgical items and blood products are running low, Europe regional director Hans Kluge told the briefing.

The supply of oxygen, children's vaccines, especially against polio amid an outbreak, and mental health expertise are among the WHO's top priorities for the region, he said.

In a separate statement, the European Center for Disease Prevention and Control (ECDC) said countries neighboring Ukraine that are hosting Ukrainian refugees should include them in their vaccine programs against a range of diseases,

They should prioritize vaccinating against COVID-19 and polio and also against measles, as the current inoculation coverage is insufficient to prevent measles outbreaks, the ECDC said in a statement.

"Crowding in bomb shelters and reception centers could facilitate the start of a measles outbreak, particularly as spring coincides with the natural seasonality of the disease," the agency added.