KINSHASA, Feb 21 (Reuters) - At least 38 Rwandan militiamen and five Congolese soldiers have been killed in clashes this week as Congo's government strives to impose its authority on the country's war-torn east, a U.N. official said on Wednesday.

Major Ajay Dalal, spokesman for the U.N. peacekeeping mission in North Kivu province, said the fighting erupted after Democratic Republic of Congo's army deployed a battalion last week made up of reintegrated combatants from a 1998-2003 war.

"The mixed brigade started patrolling and some of their patrols met with FDLR (Democratic Forces for the Liberation of Rwanda) and there were some clashes," Dalal told Reuters by telephone.

Citing casualty figures provided by the Congolese army, he said at least 38 militia fighters had been killed during the past two days, compared with only five army dead.

The U.N. mission in Congo (MONUC) had not been able to independently verify these figures, he added.

The fighting took place in jungle some 30 km (19 miles) northeast of the North Kivu town of Rutshuru, close to Congo's eastern frontier with Rwanda and Uganda.

"MONUC is now patrolling the area," said Dalal. "The present situation is calm but volatile."

The area borders the giant Virunga National Park, a UNESCO world heritage site which has become a hide-out for Rwandan militia and their Congolese Mai-Mai counterparts.

President Joseph Kabila, who took office in December as Congo's first democratically elected leader in more than four decades, has pledged to bring peace after a 1998-2003 war, which killed 4 million people through violence, hunger and disease.

During the conflict, six neighbouring countries including Rwanda invaded the giant central African state to plunder its massive reserves of diamonds, timber, gold and coltan, a metal used in the production of mobile phones and laptops.

Bands of foreign and Congolese militia still terrorise the country's east, despite the presence of the world's largest U.N. peacekeeping mission. Congo's new army is itself identified by rights groups as the worst abuser of human rights in the area.

The FDLR is largely made up of former Rwandan soldiers and members of the Hutu militias, or Interahamwe, which took part in Rwanda's 100-day genocide in 1994 in which more than 800,000 people were killed.

They fled Rwanda when a Tutsi force swept to power in July that year, led by current Rwandan President Paul Kagame.

Kigali has long accused Congo of arming and harbouring the FDLR, but Kinshasa has made efforts in recent years to expel the militia, prompting the resumption of diplomatic relations last year.

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