(Adds comment from U.N. mission chief)

By Joe Bavier

KINSHASA, Sept 21 (Reuters) - Congo's U.N. peacekeeping mission struggled on Sunday to impose a temporary ceasefire as fighting between government forces and Tutsi rebels neared an eastern city, a U.N. spokesman said.

Government forces attacked rebel positions with heavy guns and mortars in two locations on Sunday, rebels said in a statement, although there was no independent confirmation.

Rebels in hillside positions traded heavy gun and mortar fire with government soldiers below early on Saturday in the town of Sake, 25 km (16 miles) from Goma, strategic capital of troubled North Kivu province.

U.N. troops, based in Sake to monitor the town since a failed government offensive against the insurgents last December, were caught in the crossfire.

"We will not let the (rebels) take Sake," MONUC's military spokesman Lieutenant-Colonel Jean-Paul Dietrich said on Sunday.

"There is still no solution. There isn't yet a ceasefire. But at least they aren't shooting at each other right now."

Sake is located on one of two main roads leading out of Goma and just a few kilometres from camps containing tens of thousands of North Kivu's internal refugees.

Congo's peacekeeping mission, known as MONUC, has repeatedly vowed to keep the town from falling into the hands of the rebels, led by renegade Tutsi General Laurent Nkunda.

Dietrich said MONUC failed to negotiate a temporary halt to fighting in Sake on Saturday, as thousands of civilians fled.

HELICOPTER GUNSHIPS

U.N. helicopter gunships fired on rebel positions on Friday near Masisi, to the northwest, to stop their advance on the town. But Dietrich said conditions in Sake meant peacekeepers could not intervene using force to halt the rebels there.

"The two sides are too close to one another. We weren't able to use our military means," he said.

MONUC has come under increasing pressure from local officials and the government for not doing enough to pressure Nkunda's rebels to disband and reintegrate into the army.

President Joseph Kabila's government signed a peace deal with Nkunda and more than a dozen other armed groups in January in the hope of ending a decade of violence that has carried on well after the official end of a 1998-2003 regional war.

However, the process has been plagued by boycotts and daily ceasefire violations. Heavy fighting resumed on Aug. 28.

On Friday, Defence Minister Chikez Diemu reaffirmed Kinshasa's commitment to the January peace process.

Nkunda's movement wants direct talks with the government and foreign mediation before it will return to the process, but the United Nations mission chief Alan Doss rejected their demands.

"We don't want to create another framework because the accords which were signed cover everything. We have the accords and they must be respected," he told Radio France International.

"We don't want to spill blood, but we have a responsibility to the inhabitants who are under threat," Doss said.

Before the latest clashes, over 850,000 people had been forced from their homes by fighting in North Kivu since 2006.

It is one of the world's worst conflict-driven humanitarian disasters, adding to the estimated 5.4 million death toll from fighting, hunger and disease since Congo's war began in 1998. (Editing by Alistair Thomson and Elizabeth Piper)

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