* UN targeted by Congolese protesters after fall of Goma
* M23 does not enjoy broad support in Congo - UN
* Rebels showed "surprising firepower" in Goma seizure
KINSHASA/UNITED NATIONS, Nov 21 (Reuters) - The United Nations defended its failed effort to prevent rebels from seizing the Congolese city of Goma, saying on Wednesday its helicopters had fired hundreds of rockets at rebels but were unable to beat them back as their ranks swelled.
Advancing M23 rebels seized the North Kivu capital on Tuesday after Democratic Republic of Congo soldiers fled.
Goma is a regional headquarters of a U.N. force, known as MONUSCO, tasked with assisting government troops to protect civilians, but peacekeepers largely looked on once the army fled and the city fell.
"Clearly, it is not the mandate of MONUSCO to directly hit the armed groups ... They have to be in support of the armed forces of Congo," U.N. peacekeeping chief Herve Ladsous told reporters at the United Nations. "That is hardly achievable in a situation when there is no FARDC (Congo army) to support."
France called the failure of the peacekeeping force, which has heavy weapons and helicopters, to halt the advance of the lightly-armed rebels "absurd," and said its mandate should be reviewed.
Roger Meece, head of the U.N. mission in Congo, briefed the Security Council on Wednesday by video link from Kinshasa, and said the rebels has shown renewed capacity and sophisticated tactics as they advanced on Goma over the weekend.
While U.N. officials have said M23 were not engaging U.N. peacekeepers now, Meece said the United Nations had been the target of Congolese protesters.
"Since the occupation of Goma by the M23 we find there are violent protests and spontaneous demonstrations aimed at symbols of power in the Congo as well as U.N. staff and facilities," he said. "The risk of seeing this spread to other cities in the Congo is not to be excluded."
Meece added: "The M23 does not enjoy broad support in North Kivu, or elsewhere in eastern DRC. Despite strong efforts, it has been unable to establish a base of support in South Kivu or elsewhere beyond its core area of occupation."
U.N. helicopter gunships flew 17 sorties, firing 500 rockets and four missiles in the defence of Goma, the United Nations said in a statement on Wednesday giving its account of the battle. Two South African peacekeepers were injured.
About 500 rebels made an initial advance on Nov. 15. About 64 rebels were killed by army forces backed by U.N. helicopter gunships, it said. But two days later, the rebels returned in far greater numbers, launching a new attack with 3,000 men.
"Facing this new offensive ... which had a surprising firepower, Congolese forces were determined in the defence of their positions before pulling back ... despite support from their and our attack helicopters," the U.N. statement said.
The sudden increase in the size and capability of the rebel force is likely to add to accusations the M23 are receiving backing from abroad. Congo and U.N. experts have accused Rwanda of supporting the rebels, a charge Rwanda rejects.
The U.N. Security Council has expressed "concern at reports indicating that external support continues to be provided to the M23, including through troop reinforcement, tactical advice and the supply of equipment," but did not name Rwanda.
The rebellion was launched eight months ago by mutinous troops accusing the government of failing to stick to a 2009 deal with insurgents to end a previous conflict. On Wednesday they said their aim was to "liberate" the country.