ABIDJAN – French and UN helicopters fired on the Ivory Coast presidential palace and bases of Laurent Gbagbo Monday as elected president Alassane Ouattara launched an all-out push to make him step down.

The helicopters targeted the presidency, Gbagbo's residence and military barracks in the main city Abidjan, a UN official said, hours after fighters for Ouattara launched a new offensive after a months-long election dispute.

Heavy artillery fire and explosions shook downtown Abidjan as the offensive raged into the evening, AFP journalists reported.

Helicopters with the UN mission, UNOCI, "fired on the Agban and Akuedo military camps as well as the palace and presidential residence," mission spokesman Hamadoun Toure said.

"We are working with the French force Licorne, in line with our mandate and the UN resolution 1975," he added.

The resolution adopted March 30 ordered sanctions against Gbagbo in a bid to make him give up power after he lost November elections and also said the UN force should protect civilians and prevent use of heavy weapons.

The joint military action followed an "urgent" request for help by UN Secretary General Ban Ki-moon in a letter to French President Nicolas Sarkozy, officials said.

"It is urgent to launch necessary military operations to put out of action the heavy arms which have been used against the civilian population and the peacekeepers," Ban wrote, according to a copy of the letter obtained by AFP.

Gbagbo's forces had "intensified the use of heavy weapons against the civilian population," he said.

Patrols from the UNOCI peacekeeping mission had faced similar attacks when protecting civilians and convoys carrying the wounded, he said.

Ban insisted the helicopter raids did not mean the global body had taken up arms against Gbagbo.

"Let me emphasize that UNOCI is not a party to the conflict," Ban said in a statement. "In line with its Security Council mandate, the mission has taken this action in self defense and to protect civilians."

But one of Gbagbo's advisors in Paris, Toussaint Alain, said the strikes were "illegal" and amounted to an "assassination attempt" on Gbagbo.

The French military said its action "aimed to neutralise heavy weapons positions in barracks and armoured military vehicles equipped with cannons and rocket launchers".

An AFP journalist saw four Licorne helicopters firing on the Agban military camp while other witnesses reported UN helicopters shooting on the Akouedo barracks.

Outtarra's offensive began in the early afternoon.

"At 13H00, movements started towards four large corridors. We are securing our passage. The objective is to converge on Plateau and Cocody (north)," said Sidiki Konate, spokesman for Ouattara's prime minister Guillaume Soro.

Gbagbo's residence is in Cocody.

Amid the tensions Monday, armed men kidnapped several people, including two French nationals, from a hotel in Abidjan, the French foreign ministry said.

Residents of the city of five million were in lockdown in their homes the whole day while armed men patrolled the streets.

About 250 foreign nationals were flown out of the city to neighbouring countries after 167 left on Sunday, according to the French military which had taken over control of the airport in Abidjan and boosted its troop numbers.

Weary of failed diplomatic efforts to resolve the post-election crisis, Ouattara's army launched a lightning offensive further north a week ago, seizing much of the country and entering Abidjan late last week.

Gbagbo was weakened by the desertion of key allies and isolated by the international community when the battle for Abidjan began but later clawed his way back, managing to repulse attacks on his strongholds.

Reports have meanwhile emerged of human rights abuses over the week including the massacre of hundreds of civilians in the western town of Duekoue.

The International Red Cross has said 800 died in Duekoue in one day in an incident "particularly shocking by its size and brutality" while the UN mission gave an initial death toll of 330, saying that while both camps were involved.