LONDON — Burundi’s environment minister was shot to death while en route home from a New Year’s celebration early Sunday morning, an act that could exacerbate the country’s nearly two-year political crisis.

Emmanuel Niyonkuru, 54, the minister for water, environment and planning, was killed around 12:45 a.m. while returning to his home in the Rohero section of Bujumbura, the capital, according to a statement by Pierre Nkurikiye, a spokesman for the national police. He said that a woman who was with Mr. Niyonkuru had been detained for questioning.

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President Pierre Nkurunziza called the killing an “assassination” and said on Twitter: “Condolences to the family and to all Burundians. This crime will not go unpunished.”