BANGUI, June 12 (Reuters) - The United Nations suspended humanitarian operations in the impoverished northwest of Central African Republic (CAR) on Tuesday, a day after a French aid worker was shot dead in the remote strife-torn region.

Elsa Serfass, 27, a worker for medical charity Doctors Without Borders, was gunned down in her car near the town of Paoua during a trip to assess sanitary conditions after attacks by rebel and army troops last month.

"I condemn the killing of any humanitarian worker and have suspended all United Nations movements in Paoua until we get a clearer picture of what happened," said Toby Lanzer, U.N. humanitarian coordinator in the former French colony.

"Operations throughout the rest of CAR will continue for the time being," he said in a statement.

Plans to open a U.N. bureau in Paoua -- one of four new offices aimed at coping with a rising number of refugees in Central African Republic -- had been suspended because of the attack, a U.N. spokeswoman said.

Ranked as the sixth poorest country on earth in the U.N. development index, Central African Republic has been riven by decades of conflict and military uprisings since independence from France in 1960.

President Francois Bozize, who seized power in a 2003 coup, is battling separate rebellions in the northeast and northwest. The fighting has displaced more than 200,000 people in the north of the landlocked country, from a total population of 4 million.

In addition, Central African Republic also hosts 10,000 refugees fleeing a rebellion in eastern Chad and the humanitarian crisis in Sudan's western Darfur region, where more than 200,000 people have been killed in four years by fighting between rebels, the army and pro-government Arab militias.

In the northeast, U.N. agencies have mounted an urgent operation to rush food and basic supplies to 2,650 Sudanese refugees who trekked more than 200 km (125 miles) by foot to flee fighting in southern Darfur last month.

Lack of clean drinking water in the town of Sam Ouandja meant many refugees were falling ill with diarrhoea, while others were suffering from malaria.

A U.N. convoy carrying 80 tonnes of food, seeds and agricultural tools, water purification sets and medical supplies left for Sam Ouandja on Sunday but is expected to take at least 10 days to arrive because of poor roads.

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