Belarus foreign minister Vladimir Makei has reportedly died aged 64.
Belarusian state news agency Belta reported the “sudden” death – but no cause was mentioned.
It said: “Foreign minister Vladimir Makei has passed away suddenly.”
Belarus, which is north of Ukraine, had also continued to host Russian military bases during the invasion, which started in February.
Russian foreign ministry spokeswoman Maria Zakharova wrote on Telegram: “We are shocked by the reports of the death of the head of the ministry of foreign affairs of the Republic of Belarus, Vladimir Makei.
“Official condolences will be published soon.”
Earlier this week, Mr Makei attended a conference in Armenia of the Collective Security Treaty Organisation (CSTO), a military alliance of several post-soviet states.
He was expected to meet his Russian counterpart Sergei Lavrov on Monday.
In September, he defended Belarus’s position in the Ukraine war to the United Nations Security Council, saying: “Belarus is referred to as an ‘accomplice of the aggressor’ or even a party to the conflict.
“We have said and continue to say: Belarus has never advocated the war. But we are not traitors either.
“We have allied commitments, and we are strictly following and will follow the spirit and letter of international treaties to which we are parties.”
Mr Makei echoed the Kremlin’s reasoning for the war by blaming Nato and the West, who “overlooked the legitimate security interests of both Russia and Belarus”.
Before joining the government, he had a military career in soviet Belarus from 1980 to 1993 that saw him rise to the rank of colonel.