By Patrick Nduwimana

BUJUMBURA, April 20 (Reuters) - A U.N. mission to evaluate Burundi's peace process has cancelled its visit, a U.N. official said on Sunday, after a three-day battle between rebels and government troops killed at least 26.

Army spokesman Adolphe Manirakiza said no fighting was reported on Sunday, when the U.N. team had been due to arrive in the tiny central African coffee grower.

But the atmosphere was tense in the lakeside capital Bujumbura after some of the worst fighting in months. The clashes started on Thursday when Forces for National Liberation (FNL) rebels fired mortar bombs at government positions in Bujumbura, drawing return fire from the military.

"In agreement with Burundi's government, the mission has been delayed to a further date," said Amadou Ousmane, a spokesman for the United Nations' mission in Burundi.

"This is of course linked to the latest events in the country," he added, without saying when the U.N. team would reschedule its visit.

Burundi, a country of 8 million, is emerging from more than a decade of ethnic conflict in which 300,000 people were killed.

Despite a 2006 truce, the FNL continues to mount sporadic attacks against the military from its strongholds on the outskirts of the capital.

Implementation of the peace process stalled when the FNL quit a truce monitoring team in July, accusing mediators of bias.

U.N. Secretary-General Ban Ki-moon has condemned the latest fighting, saying it "inflicts unnecessary suffering on the civilian population and jeopardizes the implementation of the September 2006 peace agreement".

The clashes also forced Belgian airline SN Brussels to cancel its expected Sunday night flight to Bujumbura. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Editing by Katie Nguyen and Mary Gabriel)

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