KINSHASA (Reuters) - Ugandan Lord's Resistance Army (LRA) rebels killed at least 290, and maybe more than 300 people in Congo in a previously unreported massacre in December 2009, U.N. officials told Reuters on Saturday.

The killing spree took place in villages in Democratic Republic of Congo's remote northeast and followed warnings of rebel threats after similar massacres the year before.

"The men were tied by the chest by the same rope and killed with wood sticks on the back of the head and neck -- it was really brutal and fast," said the United Nations' Liliane Egounlety, who led the investigation into the killings in the Haut-Uele district.

"They also used machetes. Many witnesses found it too hard to talk about."

One villager cycled 60 km (40 miles) to find a phone to tell the United Nations about the massacre.

News of the killings will fuel the debate over the role and future of the much-criticised U.N. mission, which complains it lacks resources to protect civilians but is also under pressure from the government to pull out of Congo by next year.

The United Nations has a base at Niangara, about 50 km (30 miles) to the east, though there are fewer than 100 troops and no helicopters.

"We have confirmed 290 at least have been killed and 150 abducted," said Egounlety, whose team interviewed 31 witnesses in Tapili, one of the villages where the massacre took place.

Todd Howland, director of the joint U.N. human rights office in Congo, said the number "could easily reach over 300".

The LRA fought a two-decade long insurgency in northern Uganda before crossing into Congo in 2005. Its jungle bases were then attacked by a Ugandan-led multi-national force in late 2008, and the LRA rebels have splintered into groups.

Most of the fighters crossed into Sudan and Central African Republic, where they have carried out waves of attacks but experts think one group remains based in Congo.

Howland said it had taken so long for the United Nations to carry out the investigation because the area was remote and had no mobile phone network coverage. U.N. vehicles struggled to reach the site and helicopters could not land.

The United Nations sent a research team in January and again this month, and also drew on information from the local Red Cross, national army and non-governmental organisations.

TAKING THREAT SERIOUSLY?

Most of the U.N. mission's 22,000 troops are stationed in the east of the country, where a U.N.-backed operation to oust Rwandan Hutu rebels is taking place.

About 4,000 are scattered throughout the rest of the country, which is the size of western Europe and is still recovering from a 1998-2003 war that killed millions.

Despite fighting in the east and north, Congo has asked the U.N. soldiers to withdraw next year, during which presidential elections are due to be held.

Howland questioned whether the international community was taking the threat to civilians "sufficiently seriously".

"The government is asking the peacekeeping mission to leave and the international community is thinking that might be acceptable. In another situation we would have sent peacekeepers straight in," he said.

"The reason (it carries on) is that the LRA doesn't threaten anybody -- they don't threaten the government in Kinshasa or Kampala. It threatens the people in that particular place, and they are not significant numbers of voters."

In December, the United Nations said the LRA had killed 1,200 people and abducted 1,400 others -- including 630 children and over 400 women -- in a 10-month period in Congo throughout 2008 and 2009.

"We put out a report in December calling into question what the international community is doing, and then this massacre happened after that," Howland said.

"The international community needs to be more robust - the minimum objective is to provide proactive protection," he said.