From our information, one in two Kyiv residents has left the city, the mayor told Ukraine television.

"From our information, one in two Kyiv residents has left the city," he told Ukraine television. "A little less than two million people have currently left. However, Kyiv has been transformed into a fortress. Every street, every building, every checkpoint has been fortified."

The greater Kyiv area had a population of 3.5 million last year, according to website citypopulation.de.

Russian forces have rolled their armoured vehicles up to the northeastern edge of the city, an AFP team saw on Thursday.  Ukrainian soldiers described a night of heavy battles for control of the main highway leading into Kyiv.

The UN estimates more than 2.3 million refugees have left Ukraine since Russia's invasion on Feb.24, which prompted unprecedented Western sanctions against Moscow along with a cultural and sporting boycott.

At least 35,000 civilians were evacuated from the cities of Sumy, Enerhodar and areas around Kyiv on Wednesday, President Volodymyr Zelensky said, with three more routes set to open up Thursday, including out of the southern port city of Mariupol.

In addition to the growing number of refugees, at least 1 million people have been displaced within Ukraine, International Organisation for Migration (IMO) director general Antonio Vitorino told reporters. The scale of the humanitarian crisis is so extreme that the "worst case scenario” in the IOM's contingency planning has already been surpassed, he said.

Russian- and Ukrainian-speaking trained psychologists are badly needed, Vitorino said, as more traumatised witnesses join those fleeing.

Nationwide, thousands of people are thought to have been killed across Ukraine, both civilians and soldiers, since Russian forces invaded two weeks ago. City officials in the blockaded port city of Mariupol have said 1,200 residents have been killed there, including three in the bombing of a children’s hospital.

In Ukraine’s second largest city, Kharkiv, the prosecutor’s office has said 282 residents have been killed, including several children.

The United Nations human rights office said on Wednesday it had recorded the killings of 516 civilians in Ukraine in the two weeks since Russia invaded, including 37 children. Most have been caused by "the use of explosive weapons with a wide impact area,” it said. It believes the real toll is "considerably higher” and noted that its numbers don’t include some areas of "intense hostilities,” including Mariupol.

Some of the latest refugees have seen those deaths first-hand. Their testimonies will be a critical part of efforts to hold Russia accountable for targeting civilians and civilian structures like hospitals and homes.

US Vice President Kamala Harris on Thursday embraced calls for an international war crimes investigation of Russia, expressing outrage over the bombing of the children’s hospital in Mariupol. "Absolutely there should be an investigation, and we should all be watching,” she said.

Some countries continued to ease measures for refugees. Britain said that from Tuesday, Ukrainians with passports no longer need to travel to a visa application center to provide fingerprints and can instead apply to enter the UK online and give fingerprints after arrival. Fewer than 1,000 visas have been granted out of more than 22,000 applications for Ukrainians to join their families there.

Agencies