The U.N. refugee agency says more than 4 million refugees have now fled Ukraine following Russia’s invasion.

The release of the figure published Wednesday is a new milestone in the largest refugee crisis in Europe since World War II. The figure makes up nearly 10% of Ukraine’s population before the war.

The new figure was posted on the UNHCR website.

More than 2.3 million have arrived in Poland but many have traveled onward to other countries or back into Ukraine.

Large fractions of refugees have also fled to Romania, Moldova, Hungary and Slovakia.

Aid workers say the numbers have eased in recent days as many people await developments in the war.

The UN’s commissioner for refugees said 13 million people are believed to be stranded in affected areas or are unable to leave.

An estimated 6.5 million people have also been displaced from their homes within the country.