* Congolese officials resume Goma posts, army returning
* U.N. experts see "direct" Rwanda hand in M23 seizure of Goma
* Situation uncertain in absence of definitive peace deal
* U.N. says thousands are displaced in fear of more fighting
By Ed Stoddard
GOMA, Democratic Republic of Congo, Dec 3 (Reuters) - Congo's government re-established control over the eastern city of Goma on Monday after rebels withdrew, and U.N. experts made new allegations that Rwandan soldiers took part in the insurgents' capture of the city.
The M23 rebel movement pulled its fighters out of the North Kivu provincial capital in eastern Democratic Republic of Congo (DRC) on Saturday after seizing it from fleeing U.N.-backed government forces and holding it for 11 days.
But the situation remained tense and uncertain in the absence of any definitive peace accord to end the eight-month-old insurgency, which has displaced thousands of civilians in a region that is a tinderbox of ethnic and political conflict.
In a development likely to stoke diplomatic tensions, a group of experts tasked by the U.N. Security Council has presented new evidence alleging M23 received "direct support" from the Rwandan Defence Force (RDF) to capture Goma on Nov. 20.
The allegations, made in a Nov. 26 letter to the Security Council, said RDF units operated alongside M23 fighters in the rebels' advance on Goma. It added that on Nov. 20, "a mixture of M23 and RDF troops clandestinely entered into Goma from the Rwandan town of Gisenyi through small streets situated between the town's two official border crossings".
Security Council diplomats confirmed to Reuters the authenticity of the experts' letter.
Rwanda has strongly and repeatedly rejected previous allegations made by the same U.N. experts that the Rwandan government has created, equipped, trained and directly commanded the M23 rebellion in Congo's North Kivu. Similar accusations against Uganda's government have also been denied by Kampala.
In Goma on Monday, North Kivu Governor Julien Paluku, who had left when the rebels took over, and Congolese Interior Minister Richard Muyej were supervising the resumption of government operations and authority over the city.
Goma sits among lush green hills around Lake Kivu on the border with Rwanda and its capture triggered an international diplomatic scramble to head off any escalation of the fighting.
Under a deal brokered by Uganda days after Goma's fall, M23 leaders agreed to withdraw to positions 20 km (13 miles) north of the city after DRC's President Joseph Kabila said he was ready to listen to the rebels' grievances.
But Muyej said some M23 units were much closer to the city than had been agreed. "They are in Monigi. It is only 3 or 5 km away. It is not good," he told Reuters.
M23 spokesman Amani Kabasha told Reuters by telephone some rebels were in Monigi, which is on the road north to Rutshuru.
But he said the fighters there would form part of an M23 detachment that would join government troops (FARDC) and a neutral international force to be stationed together at Goma airport - one of the points agreed in the withdrawal deal.
THOUSANDS HAVE FLED FIGHTING
M23 draws most of its strength from Tutsi former rebels who had been integrated into Congo's national army but mutinied in April and started a fresh rebellion.
It has called for talks between Kabila and political opponents, the release of political prisoners and dissolution of DRC's electoral commission, which oversaw Kabila's re-election in 2011 in a vote judged flawed by foreign observers.
Government spokesmen have not confirmed that Kabila is willing to hold such a wide dialogue, and the president faces pressure from within his own armed forces, known as the FARDC, to pursue a military solution against M23.
The Nov. 26 letter from the U.N. experts said that when M23 began its offensive on Goma "it benefited from direct RDF support during combat on the frontlines at the village of Kibumba, according to former RDF officers, FARDC officers and local leaders".
The same sources estimated "well over" 1,000 RDF troops came from Rwanda to assist M23 in these operations, the letter said.
Two days after the rebels' withdrawal, Goma's dusty streets were busy, with markets selling vegetables and smoked fish, and roads choked with traffic. But banks remained closed.
Some FARDC soldiers were already back in the city barracks. They milled around the tents and dilapidated buildings and a few smoked, sitting on the back of a vehicle.
Goma lies at the heart of Congo's eastern borderlands which have suffered nearly two decades of conflict stoked by long-standing ethnic and political enmities and fighting over the region's rich resources of gold, tin, tungsten and coltan - a precious metal used to make mobile phones.
Successive attacks by rebels, militias and government soldiers have made the region notorious for mass killings, recruitment of child soldiers and rape used as a weapon of war.
The U.N. humanitarian agency OCHA said at least 130,000 people were displaced and in sites and camps in and around Goma.
U.N. officials said a camp about 15 km outside Goma had been raided by unidentified gunmen late on Friday. Several women were raped and food and supplies stolen.
Thousands more civilians were fleeing attacks by armed groups which were on the rise in other areas, such as Masisi, U.N. humanitarian coordinator in DRC, Moustapha Soumare, said.
Neighbouring Rwanda has twice invaded its western neighbour over the past two decades, at one point igniting a conflict dubbed "Africa's World War" that drew in several countries.
Aid agencies say more than 5 million people have died from conflict, hunger or disease in DRC since 1998.
Kigali has justified its interventions by arguing it was forced to act against Rwandan Hutu fighters who had fled to DRC after the 1994 Rwandan genocide that saw 800,000 Tutsis and moderate Hutus killed by Hutu soldiers and militia.
Rwanda's military said on Sunday that Rwandan FDLR rebels crossed the border from DRC and attacked a game warden camp, killing one warden in what it said was the second raid by the Hutu group in six days.
The M23 rebels said they took up arms over what they call the government's failure to respect a March 23, 2009 peace deal.