(Adds details, quotes, background)

By Patrick Nduwimana

BUJUMBURA, April 18 (Reuters) - Rebels from Burundi's last active guerrilla group fired mortar bombs at government positions in clashes that killed 10 rebels and four soldiers, an army spokesman said on Friday.

The fighting, which started late Thursday and continued into Friday morning, was centred on the rebel stronghold of Bubanza some 50 km (30 miles) northwest of the capital Bujumbura, the military's deputy spokesman Colonel Justace Ciza said.

Despite a 2006 peace deal, the rebel Forces for National Liberation (FNL) have continued to mount sporadic attacks after talks to implement the accord were suspended last July when the FNL quit a truce monitoring team, accusing mediators of bias.

"FNL rebels shelled mortar bombs and threw hand grenades on our different positions ... but we retaliated," Ciza said.

"We deplore the death of four soldiers. On the rebel side, 10 of them were killed, and ... five guns seized."

A spokesman for the Hutu fighters denied the report and accused the army of provocation.

"We were attacked by government troops who were deployed in a large number to our different positions. This was self- defence," said FNL spokesman Pasteur Habimana from Dar Es Salaam.

Last month 20 people, mainly soldiers and rebels, were killed in fighting on the outskirts of Bujumbura.

The FNL's persistent insurgency is regarded by many as the final barrier to lasting stability in the tiny coffee-growing nation, where 300,000 people were killed in more than a decade of ethnic conflict.

In a sign of its increasing frustration, the government called on foreign powers to take tough measures against the FNL.

"It is clear the FNL doesn't want peace. We demand especially the United Nations and regional countries to take action against the FNL," said government spokeswoman Hafsa Mossi, without elaborating.

She said there were indications the FNL were receiving support from the Democratic Forces for the Liberation of Rwanda (FDLR) -- Rwandan Hutu rebels based in east Congo.

"According to information received by the army, these weapons (seized Thursday night) are used by the FDLR rebels," Mossi added.

The FDLR includes former Rwandan military and Hutu Interahamwe militia responsible for carrying out Rwanda's 1994 genocide in which 800,000 minority Tutsis and moderate Hutus were slaughtered. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Writing by Katie Nguyen; editing by Philippa Fletcher)

AlertNet news is provided by