Nearly five million Ukrainians have been registered as refugees across Europe since Russia launched its invasion less than four months ago, the United Nations said Thursday.

"The Ukraine war has caused one of the largest human displacement crises in the world," said UNHCR, the UN refugee agency.

An update of the agency's data portal on the Ukraine situation, aimed to better reflect movements to and from the war-ravaged country, showed that a total of 4,816,923 Ukrainians had been registered as refugees across 44 European countries since February 24.

Far more will have actually left the country, with UNHCR data showing that more than 7.3 million border crossings out of Ukraine had been recorded by June 7.

Another 2.3 million crossings had been registered back into the country, the data showed.

UN officials have said some people have gone back and forth for various reasons, including visiting families, checking their properties, helping others to escape or returning to their jobs.

The UN says women and children account for 90 percent of those who have fled abroad, with men aged 18 to 60 eligible for military service and unable to leave.

Beyond those who have fled out of Ukraine, the UN's International Organization for Migration estimates that more than eight million people have been displaced within the devastated country.

Spread across Europe 

More than half of the crossings out of Ukraine were into neighbouring Poland, which has registered 3.8 million arrivals, as well as nearly 1.7 million crossings into the war-torn country.

A full 1.15 million Ukrainians have registered as refugees in Poland, UNHCR said.

Hundreds of thousands have also crossed into other neighbouring countries like Hungary, Romania, Slovakia and Moldova, with tens of thousands registering as refugees in each of those countries.

Most, however, have moved on to other countries across Europe, with some 780,000 Ukrainian refugees registered in Germany, nearly 367,000 in the Czech Republic, and over 118,000 in Spain.

Tens of thousands have also been registered in other European countries.

"The outpouring of solidarity in states receiving refugees remains extraordinary," UNHCR said.

It hailed in particular the European Union member states which have rapidly activated the bloc's Temporary Protection Directive for the first time, providing special protection and services for Ukrainian refugees.

Of the 4.8 million registered refugees across Europe, 3.2 million of them have registered for that status or other similar national protection schemes, the agency said.

The UNHCR data portal also showed that more than 1.1 million crossings from Ukraine into Russia, and close to 17,000 into Belarus, a Russian ally, since the invasion began.

No data were available on returns to Ukraine from these countries.