WASHINGTON, June 6 (Reuters) - The White House on Tuesday said it could not say conclusively what caused the destruction of a massive dam in Ukraine, but was assessing reports that the blast was caused by Russia, which has been occupying the dam since last year.

Spokesman John Kirby said it was clear that the destruction of the dam on the Dnipro River that separates Russian and Ukrainian forces in southern Ukraine had likely caused "many deaths" and the evacuation of thousands of Ukrainians.

Kirby told reporters the damage could have a devastating impact on Ukraine's energy security.

"We've seen the reports that Russia was responsible for the explosion at the dam," he said. "We're doing the best we can to assess those reports, and we are working with the Ukrainians to gather more information, but we cannot say conclusively what happened."

Ukraine accused Russia of blowing up the Nova Kakhovka dam while the Kremlin said it was Ukraine that had sabotaged the dam.

Kirby said it was too soon to discuss any potential impact on Ukraine's long-planned counteroffensive.

He noted that Russian forces had illegally occupied the dam and controlled it at the time of the blast, but said U.S. officials had not determined whether it was an intentional act.

Asked if the destruction of the dam would constitute a war crime, Kirby said international law forbade the destruction of civilian infrastructure.

He said U.S. officials would continue to work with humanitarian partners on the ground to supply aid to those affected.