"@WHO has confirmed several attacks on health care in #Ukraine, causing multiple deaths and injuries. Additional reports are being investigated. Attacks on healthcare facilities or workers breach medical neutrality and are violations of international humanitarian law," Tedros Adhanom Ghebreyesus tweeted.

Russia is currently invading Ukraine.

On Saturday, the WHO published six reports of attacks on healthcare in Ukraine.

"WHO strongly condemns these attacks that caused 6 deaths and 11 injuries," it added.

The Guardian quoted Ukraine's emergency services as reporting casualties from a Russian missile strike which hit residential homes in the Zhytomyr region, about 140km north-west of Kyiv.

In a separate attack in Korosten, a town just south of Ovruch, one person was killed and two injured in an overnight rocket attack, the agency reported.

Meanwhile, UNHCR head Filippo Grandi said, "This is the fastest-moving refugee crisis we have seen in Europe since the end of the second world war."

According to the United Nations, more than 1.3 million Ukrainians have fled the country since Feb 24.