KYIV, Ukraine - A close adviser to the commander-in-chief of the Ukrainian army was killed when a grenade among his birthday gifts exploded on Monday, officials said.

"Under tragic circumstances, my assistant and close friend, Major Gennadiy Chastiakov, was killed... on his birthday," General Valery Zaluzhny said on Telegram, saying that an "unknown explosive device detonated in one of his gifts".

Zaluzhny was showing his son a box with grenades inside that he had received as a gift, Interior Minister Igor Klymenko said in a statement on Telegram.

"At first, the son took the munition in his hands and began to turn the ring. Then the serviceman took the grenade away from the child and pulled the ring, causing a tragic explosion," Klymenko said.

Police have identified a fellow soldier who gave the fatal gift, he said, adding an investigation was under way.

Chastiakov leaves a wife and four children, Zaluzhny said.

He added that since Russia invaded Ukraine in February 2022, Chastiakov had been "fully devoting his life to the Armed Forces of Ukraine and the fight against Russian aggression".

An investigation into the incident has been launched, he said.

Attacks targeting Ukrainian leaders have been relatively rare since Moscow invaded, but there have been several attacks on nationalists which Russia has blamed on Ukraine.

In April, a blast from a statuette rigged with explosives killed 40-year-old pro-Kremlin military blogger Vladlen Tatarsky.

The Kremlin said the attack had been orchestrated by Ukraine with the help of supporters of jailed Kremlin critic Alexei Navalny.

But observers said the bombing attack could be used to justify a further crackdown on critics.

And last August, Darya Dugina, the daughter of a prominent ultranationalist intellectual, was killed in a car bombing outside Moscow, which Russia blamed on Ukraine.

Kyiv denied the charges.