Ukrainians on Tuesday were mourning the hundreds of children killed since the beginning of Russia's invasion, hours after a fresh attack left two children wounded in the eastern Dnipropetrovsk region.

June 4 marks the UN's International Day of Innocent Children Victims of Aggression.

More than 600 children have been killed in Ukraine since the beginning of the war in 2022, according to the United Nations, with fighting wounding another 1,420 across the country.

The UN says the real figure is likely considerably higher.

Ukraine's First Lady Olena Zelenska, speaking at a World War II memorial to commemorate killed children, urged her country's allies to supply the military with more weapons to fend off Russian attacks.

"Help us save our children. We cannot measure the value of their lives by the cost of air defences," Zelenska said at the event in Kyiv, where those gathered held a moment of silence.

In central Lviv, a western Ukraine city that has been spared the brunt of fighting with Russian forces, residents hung bells and strips of white cloth on an effigy of an angel.

The Kremlin has repeatedly said its forces do not target civilians and that the supply of Western weapons to Ukraine will only prolong the suffering of its civilian population.

Earlier this week, a 12-year-old boy was among the latest children reported killed after a Russian aerial attack on the eastern Donetsk region.

A one-month-old boy and a 17-year-old boy were meanwhile wounded by Russian rockets earlier on Tuesday in the Dnipropetrovsk region, local authorities said.

Ukrainian rights ombudsman Dmytro Lubinets said that since 2014, when Kremlin-backed separatists began seizing towns and villages in eastern Ukraine, nearly 800 children have been killed.

"These are 790 killed universes and grieving families," he said in a post on social media.

UN humanitarian coordinator Denise Brown at the event in Kyiv said that Russia was "blatantly disregarding" its commitments under the Convention on the Rights of the Child.

"Russia's invasion is inflicting immense physical, psychological, and emotional harm on children, disrupting their lives and jeopardising their future," she said.