KINSHASA (Reuters) - United Nations attack helicopters hit rebels positions in eastern Congo on Saturday after insurgents gained ground in heavy fighting, the U.N. said.

The clashes to the south of the town Kibumba mean the rebels have got to within 30 km (18 miles) of Goma, the closest they have been to North Kivu's provincial capital since a rebellion began there eight months ago.

North Kivu governor Julien Paluku said the army retreated to the southern outskirts of the town after M23 rebels received support from neighboring Rwanda.

Rwanda rejected the accusations, the latest in a string of charges by Kinshasa, and called on both the army and the rebels to halt the fighting as shells were landing in its territory.

More than five million people are estimated to have died from violence, hunger and disease in wars in Congo since 1998. It is the deadliest conflict since World War Two.

United Nations experts say they have evidence of Rwanda's rebel support and want the U.N. Security Council to impose sanctions on Rwandan officials as a result.

"The Rwandan army came across the border behind our troops, that's why our troops withdrew," Paluku told Reuters by telephone.

"The (rebels) are just a few kilometers away, so of course Goma is under threat, we can't hide that," he said, adding that government troops were reorganizing at Kilimanyoka, 12 km north of the city.

The U.N. peace keeping mission in Congo said army units had come under heavy weapons fire since early on Saturday morning, forcing civilians to flee and leading to U.N. attack helicopters being dispatched to strike rebels positions south of Kibumba.

"So far ten missions have been carried out by our attack helicopters," the U.N. said in a statement. The U.N. has a mandate to protect civilians and support government troops when they need it.

No casualty figures have been given by any force.

Rwanda's army has repeatedly sent soldiers into Congo during nearly two decades of conflict in Africa's Great Lakes region but Kigali has vehemently denied Congolese and U.N. accusations of support for the M23.

"These are absolutely false allegations. They are very tired, and very old. Whenever DRC (the Democratic Republic of Congo) is defeated on the battlefield it's meant to be (Rwanda's army)," Rwandan army spokesman Brigadier General Joseph Nzabamwita told Reuters.

"Rwanda has called on (Congo's army) and M23 to stop this useless war ... Rwanda is being violated by constant bomb shells on our territory," he added.

More than three-quarters of a million people have been forced to flee their homes since the fighting began, and regional efforts to find a solution have so far failed.

M23 spokesman Vianney Kazarama told Reuters the rebels were now in control of Kibumba but said they would not advance further.

"We're stopping here, we're waiting, we're not going to Goma," he said, reiterating a call for the government to start negotiations.

Congo's government - backed by U.N. peace keepers - has been struggling to maintain control of its restive eastern provinces since April, after hundreds of troops mutinied and launched the M23 uprising.

(Reporting by Jonny Hogg; Additional reporting by Jenny Clover in Kigali; Writing by David Lewis; Editing by Stephen Powell)