By Paul-Marin Ngoupana

BANGUI, Aug 13 (Reuters) - Rebels in Central African Republic have stepped up attacks on government forces after walking out of a peace process, the government said on Wednesday.

The United Nations said it was "deeply concerned" about deteriorating security in the northwest, where tens of thousands of civilians have fled several years of violence, and called on all sides to respect agreements they have signed.

The Popular Army for the Restoration of the Republic and Democracy (APRD) rebels, active in the region, have signed a peace deal with President Francois Bozize's government but pulled out of the accord and a planned dialogue this month.

The dialogue was meant to bring together other rebels who have signed peace deals, civil society, opposition politicians and the government to try to end decades of chaos in one of the world's least developed countries.

Demand for the gold, diamonds and uranium lying beneath its soil has drawn in mining companies including French state nuclear group Areva despite ongoing violence.

"Our forces were attacked and one of our men in the unit killed by hidden gunmen while they were escorting a humanitarian convoy in the Kabo sector," said a spokesman for Minister of Defence General Guillaume Lapo.

This week's violence followed an attack on Aug. 7 when a unit ferrying pay to soldiers in the bush was ambushed, he said.

A government soldier said two colleagues had been killed in that attack, but the Defence Ministry spokesman denied this.

APRD officials could not be reached for immediate comment.

The attacks reinforce growing doubts over peace accords signed in June.

The APRD took up arms shortly after Bozize came to power in a 2003 coup that overthrew Felix Ange Patasse. Another rebel group called the Union of Democratic Forces for Unity (UFDR) also emerged in the northeast.

Bangui's weak army required help from French soldiers stationed in the country to block an advance south by the UFDR in 2006.

A coalition of five political parties have joined the rebels in walking out of the talks. The main complaint that has led to the collapse of the talks has been the preparation of an amnesty law for crimes committed during the conflict.

The United Nations, which is trying to secure support for the country, sandwiched between the higher profile crises of Chad and Sudan, is struggling to save the deal.

"The U.N. office in CAR is following the security situation in the northwest of the country with deep concern," it said in a statement late on Tuesday that called on all sides to respect agreements they had signed. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com) (Writing by David Lewis; editing by Alistair Thomson)

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