Kyiv, Ukraine - Major Gennadiy Chastyakov, a top aide to Ukrainian Commander-in-Chief Valery Zaluzhny, was killed when one of his birthday gifts exploded.

"Today, my aide and close friend died under tragic circumstances on his birthday among his relatives," Zaluzhny announced on Telegram on Monday.

"It is an indescribable pain and a heavy loss for the Armed Forces of Ukraine and for me personally," he wrote. His post said Chastyakov left behind a wife and four children.

The police said in a statement that preliminary information indicated Chastyakov was killed by a grenade. His 13-year-old son was seriously injured.

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Interior Minister Ihor Klymenko said Chastyakov had "returned home from work with gifts from his colleagues, which he began to show to his family."

"He took out a gift box with grenades inside and began to show one of the munitions to his son."

The statement said 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."

The police found five more such unexploded grenades in the apartment.

Zaluzhny recently made headlines when he told The Economist that Ukraine's defensive campaign against the Russian invasion has reached a stalemate. The general was criticized by the president's office for his detailed statements.