UN agencies on Sunday called for an immediate ceasefire and an end to attacks on healthcare professionals and facilities in Ukraine, which have killed a dozen people, describing them 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), said the joint statement.

It was signed by the heads of the United Nations Children's Fund (UNICEF), the UN Population Fund and the World Health Organization.

"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.

In 24 of the reported attacks, health care facilities were damaged or destroyed, while in five cases ambulances were hit, they added. A total of 12 people were killed and 34 injured.

The statement called for an immediate ceasefire.

Aid and health care workers had to be able to work in safety, "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," they said.

It was signed by UNICEF Executive Director Catherine Russell, UNFPA counterpart Natalia Kanem and WHO head Tedros Adhanom Ghebreyesus.

Ukrainian police released this video grab of footage taken of a damaged children's hospital in Mariupol
Ukrainian police released this video grab of footage taken of a damaged children's hospital in Mariupol National Police of Ukraine via AFP / Handout

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.

More than 4,300 births have occurred in Ukraine since the start of Russian invasion "and 80,000 Ukrainian women are expected to give birth in next three months," the UN officials said.

Already 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," the UN agencies warned. "Every effort must be made to prevent this from happening.

"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."