Authorities in Ukraine say more than 500 children have died in the country since the Russian invasion started 20 months ago.

The country's prosecutors announced Monday the conflict has left at least 508 children dead and 1,138 wounded.

They say 492 of them were injured in Donetsk and 304 in Kharkiv. Both are eastern regions.

On Sunday, a 15-year-old girl and a 17-year-old boy were wounded by Russian shelling in Kupiansk, Kharkiv.

Officials say actual child casualties are likely to be higher as they do not know how many have died or been wounded in Russian-held areas in the country.

Meanwhile, Russian forces apparently have continued their pursuit of the eastern regions despite losing many troops to Ukrainian strikes.

The Ukrainian Air Force reported Monday the Russians launched a night-time offensive using drones and missiles.

Local authorities in the southern region of Odesa said Ukraine has intercepted nine Russian drones but that shrapnel from the drones damaged port facilities.

On Sunday, US think tank the Institute for the Study of War released an assessment on the reported acceleration of Russian attacks around Avdiivka in Donetsk.

The report says Russian forces are funneling additional forces to the Avdiivka front despite ongoing challenges.

The British Ministry of Defence said in its intelligence update on Sunday recent Russian assaults in Avdiivka have contributed to a 90 percent increase in Russian casualties recorded by the Ukrainian Ministry of Defense.