UN warns Ukrainian refugee exodus is a 'massive humanitarian crisis'

The United Nations said on Thursday the refugee exodus from Ukraine was a "massive humanitarian crisis" that was growing by the second, after another 40,000 fled the country in 24 hours.

The flow of people fleeing across the western borders to escape the Russian assault on Ukraine has settled at around 40,000 a day over the past week.

UNHCR, the UN refugee agency, said 4,059,105 Ukrainians had fled across the country's borders since the February 24 invasion -- a figure up 39,818 on Wednesday's numbers, which saw the four million mark surpassed.

"We are confronted with the realities of a massive humanitarian crisis that is growing by the second," UNHCR said, noting also the millions displaced within Ukraine and the 13 million estimated to be stranded in affected areas or unable to leave.

Women and children account for 90 percent of those who have fled. Half of those are children. Ukrainian men aged 18 to 60 are eligible for military call-up and cannot leave.

UNICEF, the UN children's agency, says more than half of the country's estimated 7.5 million children have been displaced -- 2.5 million internally and two million abroad.

"As the number of children fleeing their homes continues to climb, we must remember that every single one of them needs protection, education, safety and support," said UNICEF head Catherine Russell.

The UN's International Organization for Migration (IOM) said that in addition to Ukrainian refugees, more than 204,000 non-Ukrainians living, studying or working in the country have also left.

UN human rights chief Michelle Bachelet said it was vital to extend a welcome "to all who have fled, without discrimination".

And as of March 16, some 6.48 million people were estimated to be internally displaced within Ukraine, an IOM survey showed.

"They are more than statistics. They are people whose lives have been upended by the war," the IOM said.

Before the Russian invasion five weeks ago, Ukraine had a population of 37 million in the regions under government control, excluding Russia-annexed Crimea and the pro-Russian separatist regions in the east.

Here is a breakdown of how many Ukrainian refugees have fled to neighbouring countries, according to UNHCR:

- Poland -

Nearly six out of 10 Ukrainian refugees -- 2,362,044 so far -- have crossed into Poland.

More than 1.1 million children have arrived in Poland, said UNICEF.

Many people who cross into Ukraine's immediate western neighbours travel onto other states in Europe's Schengen open-borders zone.

- Romania -

Some 616,592 Ukrainians have entered the EU member state, including a large number who crossed over from Moldova, wedged between Romania and Ukraine.

The vast majority are thought to have gone on to other countries.

- Moldova -

The Moldovan border is the nearest one to the major port city of Odessa.

Some 388,837 Ukrainians had crossed into the non-EU state, one of the poorest in Europe.

- Hungary -

A total of 368,807 Ukrainians have entered Hungary.

- Russia -

Another 350,632 refugees have sought shelter in Russia.

In addition, 113,000 people crossed into Russia from the separatist-held pro-Russian regions of Donetsk and Lugansk in eastern Ukraine between February 21 and 23.

- Slovakia -

A total of 283,824 people have crossed Ukraine's shortest border into Slovakia.

- Belarus -

Another 11,821 refugees have made it north to Russia's close ally Belarus.