Four missiles reported to have been fired at a military base in Abidjan as UN seeks to stop weapons being used to hit civilians

UN helicopters have attacked President Laurent Gbagbo's forces in Ivory Coast, destroying their weapons at four places where they had been shelling civilians, a UN spokesman said.

The helicopters fired four missiles at a Gbagbo military camp in the main city of Abidjan, witnesses told Reuters. "We saw two UN MI-24 helicopters fire missiles on the Akouedo military camp. There was a massive explosion and we can still see the smoke," one said. The camp is home to three battalions of the Ivorian army.

Hamadoun Toure, spokesman for the UN mission in Ivory Coast, said in an email: "We launched an operation to neutralise heavy weapons Gbagbo's special forces have been using against the civilian population for the last three months. We destroyed them in four locations."

President Nicolas Sarkozy's office said that French troops had been authorised to participate in the UN attack after a request for help from the UN secretary general, Ban Ki-moon.

The attack came as forces loyal to Alassane Ouattara, widely recognised as the winner of last year's election, entered Abidjan for what they claimed is a final offensive to topple Gbagbo.

Residents in two districts of north Abidjan reported lorryloads of soldiers advancing. Alain Lobognon, an adviser to Ouattara's prime minister, confirmed a general offensive began on Monday afternoon.

The French and UN offensive hit two military camps and the presidential residence and palace. Gbagbo's troops attempted to return fire.

Residents said they heard the first rocket falling on the Akouedo military barracks at around 5.45pm. Odette Konan, a hairdresser, said: "I could not see the helicopter, but I saw the fireball and the smoke in the sky before I ran and hid in my flat. The walls of my flat were shaking."

Toussaint Alain, a Paris-based spokesman for Gbagbo, called the strikes "illegal, illegitimate and unacceptable" and said they will have resulted in many casualties among families living at the military barracks. "France has gone to war against Ivory Coast," he added.

Separately, the French foreign ministry said two French nationals and several other people had been abducted in Abidjan. France's Europe 1 radio reported five people had been abducted in all from a hotel in the city's business district. The hotel, the Novotel, has been used by many journalists reporting on the crisis. Jean Fouquiere, a spokesman for the French Embassy in Ivory Coast, said: "I confirm that five people have been kidnapped at Novotel Hotel, including two French nationals and a man from Benin."

Ivory Coast was a French colony until independence in 1960. France still has about 12,000 citizens in the country. Earlier, sources told the Guardian the UN was considering using its helicopters after the UN's base in Abidjan was targeted and 11 of its peacekeepers shot and wounded.

The attacks are on heavy weapons which troops loyal to Gbagbo are using to strike civilians, including BM-21 rocket launchers. The international body has no air force, and so there is no question of an air offensive along the lines of Libya. But the UN has a Ukrainian airforce unit with three Mi-24 attack helicopters, that have already been actively deployed.

There also 1,650 French troops in Ivory Coast, a former French colony.

The UN security council is meeting tomorrow to discuss the situation.

The UN evacuated 170 civilian staff from Abidjan at the weekend. "We are fast approaching a tipping point," Choi Young-jin, the UN's top diplomat in Ivory Coast, told the BBC. "We are planning action, we can no longer condone their [Mr Gbagbo's forces'] reckless and mindless attacks on civilians and the UN blue helmets [troops] with heavy weapons."

Choi, whose own office has been hit by sniper fire, added: "We are now in a way under siege, so we cannot go out freely, [they're] targeting us with snipers, it's a deliberate shoot at the UN. For the last few days we've had 11 [peacekeepers] wounded by their gunshots. ... They are targeting the HQ, they cut off the water … and we are now in the bunker."

In all, 20 peacekeepers have been injured since the crisis triggered by last November's disputed election. Choi said the UN's 9,000 troops did not have a mandate to dislodge Gbagbo, but could respond to heavy weapons attacks against the UN or civilians. "We'll be using our air assets," Choi said. "We'll be taking action soon."

A convoy of several dozen vehicles with mounted machine guns carrying heavily armed pro-Ouattara troops entered Abidjan at midday, the first elements of a large force massed on the northern outskirts for what they called a "final assault", according to a Reuters eyewitness.

Heavy machine-gun fire and a few explosions could be heard minutes after they entered the city limits.

The commanding officer, Issiaka "Wattao" Ouattara, told Reuters he had 4,000 men with him, plus 5,000 already in the city. Asked how long he would need to take Abidjan, Wattao said: "We know when it starts, but it could take 48 hours to properly clean [the city]."

In a rare boost for Gbagbo, it was claimed that his top general had returned to the fold days after deserting.

Phillippe Mangou, his wife and five children left the South African ambassador's residence in Abidjan after fleeing there last week.

Lieutenant Jean-Marc Tago claimed: "The general is with us, and has always been with us. Our plan is to defend the institutions of the republic against all its enemies, against the rebels, against the mercenaries, against the [United Nations] and all those who are attacking the institutions of the republic commanded by President Laurent Gbagbo."

A Gbagbo spokesman, Ahoua Don Mello, said on state TV: "Phillipe Mangou met with his fellow soldiers on the ground. But we still don't know if he is willing to return at the helm of affairs. I don't have enough information about that."

He added: "I saw him at the residence of the president with his colleagues. He is going to deliver a statement in person."

On Ouattara's rival TV station, Serges Alla, a journalist, confirmed that Mangou had left the embassy and been picked up by a close collaborator of Gbagbo's.

But the journalist added: "Mangou was forced to leave the South African embassy because some of his relatives were made hostage by diehard supporters of Gbagbo, and Gbagbo militiamen were putting pressure on him, saying they would bomb his village if he doesn't show himself or doesn't return to the Gbagbo army."

Despite mass defections, Gbagbo has surprised many observers by fighting back, issuing a call to arms to his supporters, who went to his residence on Sunday to form a human shield.

The UN has raised concern over the possible involvement of fighters linked to Ouattara's forces in hundreds of killings in the west of the country – something the Ouattara camp denies.

In Britain, foreign secretary Gbagbo's spokesman, Abdon George Bayeto, told the BBC that there is an international plot against the incumbent. "When it comes to a fight, we are going to put up a fight," he said. "The president is not going to step down."

William Hague said a team was on standby to assist in an evacuation of European Union nationals if the situation deteriorates.

Hague said: "We call for an end to the violence, for defeated former president Gbagbo to step down, for all human rights abuses to be investigated, and for the International Criminal Court to investigate the crimes which appear to have taken place."

Hague added that Gbagbo's supporters in the African Union were "down to zero", but he understood there was no prospect of African military intervention.

Away from the fighting, UN investigators probing allegations that hundreds of people were massacred in an Ivory Coast town have found a mass grave with nearly 200 bodies, UN humanitarian chief Valerie Amos said on Monday.

The International Committee of the Red Cross said on Saturday at least 800 people were killed in intercommunal violence in Duekoue last week. The UN peacekeeping mission said the same day that pro-Ouattara forces had killed 220 people and pro-Gbagbo militia fighters killed over 110 people in Duekoue. Ouattara said his forces were not involved in the massacre.

Amos, who is visiting Duekoue as part of a trip to establish humanitarian needs in Ivory Coast, said in a telephone interview with Reuters she could not confirm that as many as 800 people had been killed.

"What we do have from the investigations that are being conducted by our colleagues on the human rights side of the United Nations is that they found a mass grave," said Amos, who heads the U.N. Office for the Coordination of Humanitarian Affairs.

"They already found nearly 200 bodies in that grave, and they have found bodies in other parts of the town as well."