By Stephanie Nebehay

GENEVA, Feb 22 (Reuters) - Armed groups are stealing humanitarian trucks and kidnapping their drivers in Darfur, threatening efforts to feed up to 3.2 million Sudanese, United Nations agencies said on Friday.

Bombs dropped on three Darfur towns near the Chad border this month -- part of a Sudanese government offensive to rid the area of insurgents -- have further fanned insecurity, they said.

The U.N.'s World Food Programme (WFP) said that 30 of its trucks and 18 local drivers had gone missing in the vast Darfur region so far this year. Those figures included four trucks and their drivers who were released earlier this week, it said.

"Twenty-six of our trucks and 14 drivers are still missing. Acts of banditry are on the rise. Convoys are regularly stopped by armed men demanding money from the drivers," WFP spokeswoman Christiane Berthiaume told a news briefing in Geneva.

Some 400 tonnes of WFP food have been stolen since January in Darfur. But the agency's food distributions are continuing, mainly in camps teeming with civilians who have fled fighting between the government and rebels since early 2003.

"We run the risk of not being able to maintain this necessary flow of trucks to bring food aid to Darfur, where we are helping between 2.1 million and 3.2 million people, depending on the season," Berthiaume said.

Relief agencies said this week that renewed fighting in Darfur and neighbouring Chad had forced thousands more civilians to flee and risked causing a more severe humanitarian crisis.

The Sudanese army announced a "cleansing" operation in the rebel-held mountainous region to open the way for humanitarian access and to rid it of Darfur and Chadian insurgents, whom it said were attacking civilians.

The U.N. High Commissioner for Refugees (UNHCR) said its staff had found a "high level of destruction" in Sirba, one of three towns hit by Sudanese air raids on Feb. 8 that rebels say killed 15 civilians.

"There is ongoing fighting (in Darfur) and some of these people feel safer remaining where they are now rather than moving. They also complain of just generalised banditry and lawlessness," UNHCR spokesman Ron Redmond said.

In eastern Chad, an aid convoy trying to reach the Birak border area -- where Sudanese refugees have gathered after fleeing the Darfur bombings -- had to turn back on Friday due to renewed fighting which could be heard on the Darfur side.

The UNHCR has been unable to move thousands of Sudanese refugees who have crossed into eastern Chad further from the volatile border area because of the insecurity, Redmond said. (Editing by Elizabeth Piper)

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