The United Nations said Wednesday that the number of Ukrainians who have fled the Russian invasion has crossed the four-million mark, while millions more are displaced within the country.

UNHCR, the UN refugee agency, said 4,019,287 Ukrainians had fled across their country's borders since the Russian attack began on February 24.

The daily flow of refugees heading west has remained steady at around 40,000 in recent days.

The increase of 117,574 on the total number of Ukrainian refugees given Tuesday is chiefly down to another 79,378 people recorded in Russia since UNHCR last received figures from Moscow on March 22.

The UN's International Organisation 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.

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

Before Russia's February 24 invasion, 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 - 2,336,799 so far - have crossed into Poland, according to UNHCR. 

Polish border guards put the number of crossings into the country even higher, at 2,368,000.

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

Romania

The number of Ukrainians who have entered EU member state Romania has reached 608,936, 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 to the major port city of Odessa. 

UNHCR said 387,151 Ukrainians had crossed into the non-EU state, one of the poorest in Europe. 

To reduce congestion, organised convoys leave daily from the Palanca crossing for Romania, with the most vulnerable prioritised for transfer. 

Hungary

A total of 364,804 Ukrainians have entered Hungary, according to UNHCR. 

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. 

The UNHCR said Tuesday it was aware of recent allegations of forced movements of people from Ukraine to Russia but could not corroborate such information. 

Slovakia

A total of 281,172 people have crossed Ukraine's shortest border into Slovakia, according to UNHCR.

Belarus

Another 10,902 refugees have made it north to Russia's close ally Belarus.

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