UN agencies on Sunday called for an immediate ceasefire and an end to attacks on healthcare professionals and facilities in Ukraine, describing such incidents as acts of "unconscionable cruelty".

Since the start of Russia's invasion of Ukraine, 31 attacks on health care have been documented via the WHO’s Surveillance System for Attacks on Health Care (SSA), the heads of the United Nations  Children's Fund (UNICEF), the UN Population Fund and the World Health Organization said in a joint statement.

"To attack the most vulnerable -– babies, children, pregnant women and those already suffering from illness and disease, and health workers risking their own lives to save lives –- is an act of unconscionable cruelty," they said, calling for an immediate ceasefire.

In 24 of the reported attacks, health care facilities were damaged or destroyed, while in five cases ambulances were hit. A total of 12 people were killed and 34 injured, according to the UN agencies' statement.

“Humanitarian partners and health care workers must be able to safely maintain and strengthen essential health service delivery, including immunisation against Covid-19 and polio, and the supply of life-saving medicines for civilians across Ukraine as well as to refugees crossing into neighbouring countries," said the statement, signed by UNICEF Executive Director Catherine Russell, UNFPA counterpart Natalia Kanem and WHO head Tedros Adhanom Ghebreyesus.

At least three people were killed, including a young girl, in an attack Wednesday on a children's hospital in Mariupol in southern Ukraine.

According to the United Nations' reproductive health agency two other Ukrainian maternity hospitals had already been attacked and destroyed before that strike.

"Attacks on health care and health workers directly impact people's ability to access essential health services -– especially women, children and other vulnerable groups," the UN agency chiefs said in their statement on Sunday.

"We have already seen that the health care needs of pregnant women, new mothers, younger children and older people inside Ukraine are rising, while access to services is being severely limited by the violence," the statement added.

Oxygen and medical supplies, including for the management of pregnancy complications, are running dangerously low, they warned.

“The health care system in Ukraine is clearly under significant strain, and its collapse would be a catastrophe. Every effort must be made to prevent this from happening," the UN agencies warned.

"We call for an immediate ceasefire, which includes unhindered access so that people in need can access humanitarian assistance. A peaceful resolution to end the war in Ukraine is possible.”