Hennady Chastyakov was killed as he showed the gift to his teenage son at home on Monday, officials said.

It was initially thought that the 39-year-old had been killed by a booby-trapped gift.

But interior minister Ihor Klymenko later issued a statement to clarify that it had been a "tragic explosion".

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Writing on the Telegram messaging app, Mr Klymenko said the major had returned home with gifts from his colleagues, including a box of Western-made grenades.

"The son first took the munition in his hand and began to twist the ring," Mr Klymenko said.

"Then, the serviceman took the grenade from the child and pulled the ring, causing a tragic explosion."

Major Chastyakov's 13-year-old was also seriously injured in the blast, Ukrainian media reported.

Mr Klymenko said the colleague who gave him the gift had been identified and that two more grenades of the same type were found in his office by investigators.

He said he had issued the statement so as not to "spread unofficial information."

Major Chastyakov had been an assistant to Ukraine's military commander-in-chief General Valery Zaluzhnyi, who expressed condolences to his family.

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He said: "My assistant and close friend, Major Hennady Chastyakov, was killed in tragic circumstances on his birthday in a family setting.

"An unknown explosive device went off in one of his presents."

A security source told newspaper Ukrainska Pravda that Major Chastyakov was a graduate of a military academy and fully trained in handling grenades.

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General Zaluzhnyi sparked a row over the weekend after he said there would "most likely be no deep and beautiful breakthrough" despite his country's counteroffensive against Russian forces, that was launched in June.

The comments prompted President Volodymyr Zelenskyy to reject claims that the war, which has been raging for 20 months, is at a stalemate.