By Patrick Nduwimana

BUJUMBURA, April 23 (Reuters) - Rebels in Burundi fired mortar bombs at the capital Bujumbura, hitting the residence of the Pope's envoy in the latest flare-up of violence that has killed 33 people in a week, the army said on Wednesday.

Fighters from the Forces for National Liberation (FNL), the last active rebel group in the tiny central Africa nation, launched their attack from outside the city on Tuesday night.

"We know that the FNL threw several bombs from hills around the capital," army spokesman Adolphe Manirakiza said.

There were no reports of casualties from the latest in a series of clashes and bombardments in the last week that have undermined a shaky peace process in a country whose stability is seen as crucial to the volatile Great Lakes region.

Witnesses said one mortar hit the residence of Vatican representative Paul Richard Gallagher. He was out of the country at the time, and it did not appear to be a targeted attack.

FNL said the attack was self-defence against the military.

"The army continues its harassment of our fighters using helicopters and other heavy arsenal, so what we are doing is to defend ourselves," rebel spokesman Pasteur Habimana said.

The FNL's persistent insurgency is seen by many as the final barrier to lasting stability in the coffee growing country of 8 million which is emerging from more then a decade of ethnic war that killed 300,000 people.

Despite a 2006 peace deal, FNL have mounted sporadic attacks after talks to implement the accord were suspended last July when it quit a truce monitoring team accusing mediators of bias.

Burundi has urged the United Nations and regional countries to take tough measures against the FNL rebels.

"We should oblige the FNL to give up its arrogance and attacks so that it returns to the negotiating table unconditionally," Burundi's Defence Minister Germain Niyoyankana told a meeting of military chiefs from neighbouring countries.

(Reporting by Patrick Nduwimana, Editing by Wangui Kanina)

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