By Tansa Musa

YAOUNDE, June 11 (Reuters) - Attacks by rebels in Central African Republic along Cameroon's border are hampering United Nations efforts to aid more than 25,000 refugees fleeing violence in the region, a U.N. official said on Monday.

In the latest of a series of attacks, rebels raided the frontier cattle-trading town of Ngaoui in the eastern Cameroonian province of Adamawa last week. They kidnapped 16 people, including 10 refugees, and demanded a ransom of 100 million CFA francs ($200,000).

"We are very worried by the growing activities of rebels from Central African Republic who are creating an almost permanent situation of insecurity along the border with Cameroon," Jacques Franquin, representative in Cameroon for the U.N. refugee agency UNHCR, told Reuters.

"Their frequent attacks and taking people hostage, including refugees ... mean that we cannot deploy personnel," he said.

UNHCR was planning to open an office in January at Meiganga, a town close to the border in Adamawa province, to cater for more than 25,000 refugees fleeing the conflict in northern CAR and a rebellion in eastern Chad.

"Hardly a month goes by without such an attack. That is why our sub-office in Meiganga is not yet fully operational," said Franquin.

The 16 hostages, mainly Mbororo cattle herders targeted for their wealth, were taken to a remote village 20 km (12 miles) inside Central African Republic. Only one was released after his family negotiated with the rebels, Franquin said.

Ngaoui is home to 3,000 refugees fleeing sporadic fighting in Central African Republic between the army and rebels seeking to unseat President Francois Bozize, who himself seized power in a 2003 coup in the world's sixth poorest country.

In April, the rebels snatched five people from the village of Gbafouk in the same region and only released them after a ransom of 12 million CFA francs ($24,000) was paid.

Cameroon dispatched a large contingent of troops from its rapid intervention brigade to the region at the weekend to maintain security, an aide to the governor of Adamawa province told Reuters.

Central African Republic has suffered decades of instability and military coups since independence from France in 1960. UN children's agency UNICEF estimates that more than a quarter of its 4 million people are suffering the effects of the civil war or spillover from conflicts in Chad and Sudan's Darfur region.

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