GENEVA (Reuters) - Burundi has forced the United Nations to shut its local human rights office after 23 years, U.N. High Commissioner for Human Rights Michelle Bachelet said on Tuesday.

Hundreds of Burundians have been killed in clashes with security forces and half a million have fled abroad since President Pierre Nkurunziza announced in 2015 he would run for a third term in what many saw as a breach of the constitution.

He won re-election and in 2016 Burundi suspended all cooperation with the U.N. human rights office in Burundi after a U.N.-commissioned report accused the Bujumbura government and its supporters of being responsible for crimes against humanity.

On Tuesday Bachelet said the Burundian government had declared it had made sufficient progress in human rights so the U.N. rights office in Bujumbura was no longer justified.

“It is with deep regret that we have had to close our office in Burundi after a 23-year presence in the country,” Bachelet, a former Chilean president, said in a statement.

She said advancements in human rights in Burundi had been jeopardized since 2015 when Nkurunziza announced his re-election bid, which sparked major protests and a security crackdown.

Burundi rejected the 2016 U.N. report as “lies” and when the U.N. Human Rights Council considered renewing the investigation in 2017, Burundi offered to accept U.N. rights experts instead.

But the rights council voted both to renew the investigation and to send the experts, infuriating the Bujumbura government.

Burundi subsequently threatened to prosecute the rights council’s team of investigators and accused its chairman of “selling” Africans like in the era of slavery, a comment that outraged Bachelet.

The government also threw out the three visiting U.N. human rights experts it had promised to cooperate with, accusing them of arriving unannounced and acting like spies.

U.N. human rights spokeswoman Ravina Shamdasani said senior Burundian officials who met Bachelet last week told her they were prepared to accept technical assistance such as human rights training and advice on legislation.

But Burundi had also shown “a serious lack of cooperation” with other U.N. human rights bodies in Geneva, and Burundian officials had left halfway through a review into use of torture in the country, Shamdasani said.

She said there were still credible reports of serious human rights violations in Burundi, including arbitrary killings, forced disappearances, ill-treatment, arrests and detention, and curbs on freedom of association, expression and movement.