More than 40,000 more Ukrainian refugees fled in 24 hours, the United Nations said Wednesday, as it warned of traffickers seeking to exploit vulnerable women and children on the move.

The UN refugee agency UNHCR said 4,656,509 Ukrainians had fled since Russia invaded on February 24 -- a figure up 40,679 on Tuesday's update.

Women and children account for 90 percent of those who have left Ukraine, with men aged 18 to 60 eligible for military call-up and unable to leave.

"The Ukrainian refugee crisis is a protection crisis for women and children," said UNHCR assistant high commissioner Gillian Triggs.

Nearly two-thirds of all Ukrainian children have been forced from their homes, including those still inside the country.

"It is impossible to gauge how many Ukrainian refugee women and children might have been preyed upon by traffickers. So far, known cases are thankfully few," said refugee protection chief Triggs.

"But we are on high alert and warning refugees on the risks of predators and criminal networks who may attempt to exploit their vulnerability or lure them with promises of free transport, accommodation, employment or other forms of assistance.

"While the generosity and solidarity towards Ukrainian refugees has been inspiring, states must prevent predatory individuals and criminal networks from exploiting the situation."

The UN's International Organization for Migration estimates  7.1 million people have fled their homes but are still in Ukraine.

The IOM says that in addition to Ukrainian refugees, more than 210,000 non-Ukrainians living, studying or working in the country have also left.

In total, more than a quarter of the population have been forced to flee their homes.

Before the 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 -- 2,669,637 so far -- have crossed into Poland, according to the UN.

Many people who go to Ukraine's immediate western neighbours travel on to other states in Europe's Schengen open-borders zone.

- Romania -

A total of 709,249 Ukrainians 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. 

- Hungary -

A total of 434,342 Ukrainians have entered Hungary.

- Russia -

Another 433,083 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. 

- Moldova -

The Moldovan border is the closest to the major port city of Odessa. A total of 415,850 Ukrainians have crossed into the non-EU state, one of the poorest in Europe.

Most of those who have entered the former Soviet republic of 2.6 million people have moved on.

- Slovakia -

A total of 323,020 people crossed Ukraine's shortest border into Slovakia.

- Belarus -

Another 21,852 refugees have made it north to Russia's close ally Belarus.