U.S. Army General Mark Milley, chairman of the Joint Chiefs of Staff, said he would reserve judgment as media reports emerged indicating that at the site in Izium, some bodies were found with hands tied behind their backs.

"In terms of the totality of the scale (of potential war crimes), I don't know. But I would tell you that the world will discover that. War crimes cannot be hidden, especially things like mass graves," Milley told reporters traveling with him after arriving in Estonia for a NATO gathering.

Ukrainian President Volodymyr Zelenskiy told Reuters in an interview that the mass burial site in Izium was proof of Russian war crimes and evidence was being collected.

"There is some evidence, and assessments are being conducted, Ukrainian and international, and this is very important for us, for the world to recognize this," he said.

Moscow has not commented on the mass burial site in Izium, which was a Russian frontline stronghold before Ukraine's counter-offensive forced its forces to flee.

The head of the pro-Russian administration which abandoned the area last week dismissed the accounts and accused Ukrainians of stage-managing atrocities.

Milley's visit to Estonia followed a trip to Israel, where, earlier in the day he visited Yad Vashem, Israel's Holocaust memorial to the 6 million European Jews murdered in World War Two.

Milley said he was not comparing the Holocaust, in all of its enormity, to events unfolding in Ukraine.

"But having said that, war crimes, if the evidence is there, then that's necessary to discover. And it's just a poignant reminder to us, because all of us were on a trip to Israel that I don't forget -- and no one should," he said.

Kyiv's biggest European supporters, such as Baltic states like Estonia which have long called for more military aid for Ukraine, say Ukraine's battlefield successes in its counter-offensive have demonstrated the case for more support.

But the mass burial site has also raised questioned about what other discoveries may await Ukrainian troops, who hope to seize more Russian-held Ukrainian territory.

Milley lauded Ukraine's military for seizing the "strategic initiative" from Russia -- terminology suggesting that Ukraine had momentum in a war now well into its seventh month.

But Milley was cautious about making predictions. Asked whether Ukraine would be able to retake all its territory, Milley said: "The offensives are in the early stages. We're only looking at probably at about two weeks so far."

"And it remains to be seen how far the Ukrainians can press this fight. So I think we'll have to wait and see how the fighting develops," he said.