UN helicopters attack in Ivory Coast (UPDATES) (VIDEO)

ABIDJAN, Ivory Coast — United Nations Mig-24 helicopters fired on forces backing besieged Ivory Coast President Laurent Gbagbo to knock out mortars firing on U.N. headquarters and the civilian population in Abidjan.

Residents in the country's largest city braced for a final battle between supporters of rival presidential candidates.

The U.N. has more than 7,500 troops in Ivory Coast and the peacekeeping forces stepped up their use of force against Gbagbo bases, saying they are protecting the civilian population and the U.N.'s own bases.

The U.N. withdrew non-essential staff earlier Monday amid reports of a massacre, and after 11 peacekeepers were shot in fighting recent days. The Special Representative in Ivory Coast of the U.N. Secretary General has accused pro-Gbagbo forces of "mindless" attacks on the U.N. base in Abidjan, according to the BBC.

"We are planning action, we can no longer condone their [Gbagbo's forces] reckless and mindless attack on civilians and the United Nations blue helmets with heavy weapons," the BBC quoted Choi Young-jin as saying. "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 United Nations.

"For the last few days we have 11 [peacekeepers] wounded by their gunshots. They are targeting the headquarters, they cut off the water… and we are now in the bunker."

The U.N. helicopters attacked Gbagbo's military bases in Abidjan.

France announced that it would carry out attacks on Gbagbo, at the request of the United Nations. The former colonial power in Ivory Coast, France has 1,500 troops that control the Abidjan airport.

Some 5,000 fighters backing Alassane Ouattara, the internationally-recognized president, entered Abidjan Monday in preparation for a final advance against supporters of Laurent Gbagbo, who has refused to relinquish power since elections last November.

"This is the final assault," said Apollinaire Yapi, Ouattara's spokesman, according to the New York Times. "I would say this is the general offensive we anticipated. So far the incursions have been to test Gbagbo's forces."

Ouattara's forces have met fierce resistance from Gbagbo loyalists in the city after having made rapid advances to claims large areas of the country, including the capital Yamoussoukro.

Ouattara's prime minister, Guillaume Soro, said the time is right for a "rapid offensive" against Abidjan, according to the BBC.

Gbagbo's forces hold a few centers in Abidjan including the presidential palace downtown, Gbagbo's expansive personal residence in the posh Cocody neighborhood, state television and military camps.

As residents barricaded themselves indoors, the United Nations — which oversees the force of foreign peacekeepers in the country — said it had relocated non-essential staff to the northern city of Bouake after personnel came under attack.

"We have made a temporary relocation of some of our staff to Bouake. It is a small part of the staff, mostly non-essential personnel," Hamadoun Toure, a spokesman for the U.N.'s Ivory Coast mission, told Reuters.

"Our patrols and headquarters were targeted by attacks, but we continue to work," he said.

France sent 300 troops to join the U.N. mission in the west African country over the weekend, taking control of Abidjan's airport. On Monday, France urged its citizens to relocate to the embassy or group together in a hotel for their safety.

"Our ambassador in Abidjan issued a message to the French community informing them of this decision and giving them instructions to follow," Foreign Ministry spokesman Bernard Valero said Monday, CNN reported.

Few people were venturing outdoors in Abidjan, a city of four million people, with food and other supplies beginning to run short.

“I haven’t been able to go out of my flat since Thursday,” resident Laurent Kone told Bloomberg. “I have almost nothing to eat at home anymore. I’m hungry. All the shops of my street are closed.”

There were reports that around 700 Gbagbo supporters had been deployed around the presidential compound after state television, still controlled by Gbagbo, called on people to form a human shield against Ouattara's advancing foot soldiers.

Meanwhile, more reports emerged over claims of a massacre in the western town of Duekoue in the wake of a fierce battle last week.

Aid group Caritas said about 1,000 people died or disappeared in the town between March 27 and March 29. The Red Cross said about 800 were killed on March 29 as a result of "inter-communal violence."

United Nations chief Ban Ki-moon has called on Ouattara to investigate claims his supporters were behind the incident. Ouattara denied the claims.

U.S. Secretary of State Hillary Clinton said that America was "deeply concerned" about the situation in Ivory Coast and called for Gbagbo to step down immediately.

The African Union has also urged Gbagbo to end his decade in power and spare his people any more suffering.

— Barry Neild