A flow of medical gear and equipment, including trauma kits, is reaching Ukraine to prop up a health care system grappling with a shortage of supplies amid Russia's invasion, the World Health Organization said on Monday.

Supply chains have been severely disrupted, with many distributors knocked out, some stockpiles out of reach because of military operations, supplies of medicine running low, as hospitals struggle to care for the sick and wounded, it said.

"The current estimated number of people impacted in Ukraine is 18 million, of which 6.7 million are internally displaced," the agency said in a statement. "Nearly 3 million people have fled the country."

WHO was working with partners to alleviate shortages of critical equipment and medication such as oxygen and insulin, surgical supplies, anesthetics, and transfusion kits, it added.

Items being shipped included oxygen generators, electrical generators, defibrillators, monitors, anesthesia drugs, rehydration salts, gauze and bandages.

WHO said supplies were being distributed in coordination with Ukraine's health ministry, backed by a support hub in neighboring Poland.

"The coming days and weeks will see a constant flow of medical supplies, as part of an effort to ensure people's access to essential drugs and medical care," it added.

The agency again condemned attacks on health care providers, saying it had verified 31 such attacks from the time Moscow invaded on Feb. 24 until March 11.

These led to 12 deaths and 34 injuries, with health workers accounting for eight of the injured and two of those killed. More attacks were being verified.

Russia denies targeting civilians in what it calls a "special operation" to disarm and "de-Nazify" Ukraine.