In a departing plea to President Vladimir Putin, UN rights chief Michelle Bachelet on Thursday urged him to stop Russia's attack on Ukraine and its "unimaginably terrifying" impact on civilians.

Bachelet, who steps down at the end of the month, marked the six months since the February 24 Russian invasion by insisting on accountability for serious rights violations in the conflict, some of which may amount to war crimes, she said.

"I call on the Russian president to halt the armed attack against Ukraine," the outgoing United Nations high commissioner for human rights told a farewell press conference in Geneva.

Tuesday marked "six unimaginably terrifying months for the people of Ukraine, 6.8 million of whom have had to flee their country," she said.

"Millions others have been internally displaced."

Her office has so far documented 5,587 civilians killed and 7,890 injured in the conflict, although it says the true civilian casualty toll will be far higher.

Of those casualties, nearly 1,000 are children.

Bachelet urged Moscow and Kyiv to respect international human rights and humanitarian law at all times.

"The international community must insist on accountability for the many serious violations documented, some of which may amount to war crimes," she added.

"The fighting continues, amid almost unthinkable risks posed to civilians and the environment as hostilities are conducted close to the Zaporizhzhia nuclear power plant," the former Chilean president added.

"The Zaporizhzhia plant needs to be immediately demilitarised."

The International Atomic Energy Agency, the UN's nuclear watchdog, wants to send a mission to the nuclear power plant, which is Europe's largest.

Yevheniia Filipenko, Ukraine's ambassador in Geneva, said Wednesday there were indications that the mission inspectors "could start their work... very soon and we hope that it will be able to implement its mandate effectively".