Ukraine will repatriate troops and equipment, including helicopters, involved in United Nations (UN) peacekeeping missions in Africa and Europe to bolster defences at home in the face of Russia's invasion, a UN official said.

Russian troops invaded Ukraine nearly two weeks ago, shelling, bombing and besieging its cities and towns and forcing more than 2-million people, mostly women and children, to flee.

Ukrainian President Volodymyr Zelenskiy signed a decree on Tuesday recalling all peacekeeping forces in order to assist in the war effort at home, Ukrainian media reported.

Ukraine contributes about 300 troops, police officers and staff to six UN missions, a number unlikely to drastically alter peacekeeping operations globally.

Its biggest contribution by far, to the mission in Democratic Republic of Congo known as MONUSCO, comprises eight Ukrainian helicopters and a 250-member aviation unit, a MONUSCO spokesperson said.

"We have received notification from Ukraine regarding the repatriation of their equipment and contributions to peacekeeping," the spokesperson told Reuters, adding that the pullout included all UN missions with Ukrainian involvement.

"At the moment we are evaluating what impact this retreat could have on our mission and how we can minimise it."

Ukraine also contributes a dozen or fewer police officers and other staff to UN missions in Mali, South Sudan, Sudan, Cyprus and Kosovo, the UN peacekeeping website says.

Thousands of Ukrainian civilians and foreign volunteers have signed up to fight, and Western countries have contributed equipment, but Ukraine's military capabilities remain dwarfed by those of its giant neighbour.

MONUSCO has about 12 000 troops deployed, most in Congo's mineral-rich east, where dozens of militia groups are fighting for control of land and resources.

The Ukrainian helicopters are based in the eastern towns of Goma, Bunia, Bukavu and Beni, MONUSCO said.

Russia describes its offensive in Ukraine as a "special operation" designed to "demilitarise" and "denazify" Ukraine.