Ukraine is desperate to fill the depleted ranks of its armed forces with fresh recruits and recently lowered the mobilisation age from 27 to 25.

Previously Ukrainian men aged 18 to 60 who had permanent residency outside the country were allowed to travel abroad.

They will now be subject to the same restrictions as other Ukrainian military age men, who are barred from leaving unless they meet some narrow criteria, such as on health grounds or a government-approved cultural or sporting trip.

The US Embassy in Kyiv said Tuesday it "understands that, effective June 1, Ukraine has eliminated a 'residence abroad' exception that previously allowed certain Ukrainian males aged 18 to 60 to depart the country".

Kyiv is tightening pressure on Ukrainian men living abroad who have not faced being called up to fight during the first two years of the war.

Border Guard spokesman Andriy Demchenko separately confirmed to AFP that Ukraine does not recognise dual citizenship and that men with permanent residency in another country are no longer allowed out.

"Under martial law, Ukrainian citizens who permanently resided outside Ukraine ... could travel outside Ukraine," he said.

"This possibility is now limited for them," he added, citing changes in legislation linked to military service.

The US Embassy told Ukrainians with US citizenship that they should not travel to Ukraine if they "do not wish to stay in Ukraine indefinitely".