Former Burundian government minister and spokeswoman Hafsa Mossi was shot dead by unknown assailants in the capital Bujumbura on Wednesday, police said.
"Hon. Mossi assassinated 10.30 in Gihosha", in the east of Bujumbura by, "two criminals in a vehicle," police spokesman Pierre Nkurikiye said on Twitter.
Mossi, a former journalist in her 50s who previously worked for the BBC’s Swahili Service, was a member of President Pierre Nkurunziza's ruling CNDD-FDD party.
"The assassination of Hafsa Mossi is a despicable and cowardly act," Nkurunziza tweeted.
"This is an inestimable loss for Burundi, her family and the whole EAC (East African Community)," he added.
Mossi was Burundi's information minister and government spokeswoman between 2005-2007, and at the time of her death was a member of the East African Legislative Assembly, a regional parliament.
Foreign Minister Alain-Aime Nyamitwe called her death "a great loss for the nation".
Nkurunziza's controversial but ultimately successful bid for a third term last year triggered a deadly crisis that has left more than 500 people dead and driven around 270,000 to leave the country.
Several senior military officers close to the president have been assassinated since the start of the crisis in April 2015, but Mossi, who was not regarded as a party hardliner, is the first senior politician to be killed.
She had stayed very quiet since the beginning of the crisis and the motive for her murder remained unclear.
An investigation was swiftly opened.
Louise Mushikiwabo, the foreign minister of neighbouring Rwanda, which has difficult relations with Burundi, spoke on Twitter of a "good woman and a dedicated politician. Rest in Peace my sister!"
France condemned the assassination in a statement, adding that Paris was monitoring closely the "continued violence and human rights abuses in Burundi".
The statement also called for an "extensive dialogue" to help bring about an end to the crisis engulfing the country.
Amnesty International called for an independent enquiry into Mossi's assassination.
"Hafsa Mossi’s death, the latest in a disturbing string of high-profile killings, is a great loss to the people of Burundi and the East African region,” said Amnesty's regional director Muthoni Wanyeki.
"Her murder must be promptly, independently and impartially investigated, and those reasonably suspected of responsibility held to account in fair trials.”
