ABIDJAN (AFP) - The French army took over Ivory Coast's main airport Sunday as the battle for Abidjan raged into a fourth day and rival leaders blamed each other for chilling massacres in the west. The French Licorne (Unicorn) force took control of the airport in the main city Abidjan and Paris reinforced its troops in the city with 300 men as more than 1,500 foreigners sought refuge at a French military camp amid violence and looting in the city. Fierce fighting has been raging in Abidjan and other parts of the west African country between forces backing internationally recognised president Alassane Ouattara and incumbent strongman Laurent Gbagbo, who refused to step down after disputed November elections. Following reports of massacres in the west of the country that left hundreds dead, UN chief Ban Ki-moon on Sunday demanded Ouattara take action against followers who may have taken part. In a telephone conversation with Ouattara, Ban expressed "concern and alarm" over reports of the killings in the western town of Duekoue, UN spokesman Martin Nesirky said. Ouattara denied his followers were involved in the killings but said he had ordered an investigation, Nesirky said. Gbagbo's spokesman in Paris, Toussaint Alain, also denied his troops had played any role in the massacres. "We deny the accusations of violence allegedly committed by members of the Ivory Coast army. We have no mercenaries in the west. This entire area is 90 percent controlled by the rebellion which bears the responsibility for this massacre," he said. Early Sunday residents and AFP journalists said Ouattara's fighters held firm to their positions as sporadic heavy arms fire broke out near the presidential palace. Gbagbo's whereabouts were unknown. Tension in the city reached fever pitch as residents, most fearful to leave their homes, awaited a final showdown between the rival leaders. Gbagbo, who continued resisting international calls to step down, was mobilising his supporters around his strongholds in Abidjan and his regime stepped up its virulent rhetoric against former colonial master France. The United States early Sunday called on Ivory Coast strongman Laurent Gbagbo to step down immediately, saying he was pushing the West African nation into lawlessness. "The United States calls on former President Laurent Gbagbo to step down immediately," Secretary of State Hillary Clinton said in a statement. "Gbagbo is pushing Cote d'Ivoire into lawlessness," she added. "The path forward is clear. He must leave now so the conflict may end." Alain accused Licorne of "behaving like an army of occupation." "The French army has entered into the service of the rebellion... the French army has become the rebellion's auxiliary," Alain said. Gbagbo's state television also broadcast virulently anti-French messages. "A Rwandan genocide is being prepared in Ivory Coast by Sarkozy's men," read a ticker running across the screen, referring to French President Nicolas Sarkozy and the 1994 massacre of 800,000 in Rwanda in just three months. "The French army has occupied the Felix Houphouet-Boigny airport, we are in danger," read another. The RTI television station, a vital means of communication for Gbagbo to his loyalists, was briefly captured by pro-Ouattara fighters on Thursday night before Gbagbo troops regained control and restored the signal. A message was played on Sunday in which Damana Pickas, a leader of the "Young Patriots", Gbagbo's most fervent backers, called for mobilisation. "We need all the patriots, the resistance, to take to the streets. There are times when death is better than dishonour. But do not doubt our victory," he said. All day Saturday RTI broadcast calls for mobilisation, urging civilians to form a "human shield" around Gbagbo's residence. That evening hundreds of people including women and children were shown gathered in front of his home. Weary with failed diplomatic efforts to resolve the post-election crisis, Ouattara's army has seized much of the country and is now waging its offensive in the economic capital. Fierce attacks on Gbagbo's strongholds have shaken Abidjan with artillery explosions and machine gun fire, but the strongman, internationally isolated, economically asphyxiated and virtually cornered, is fighting to the last. The reports of carnage in the west of the country have emerged from aid groups and the United Nations. The International Red Cross has said 800 died in Duekoue in one day in an incident "particularly shocking by its size and brutality". The Catholic mission Caritas reported 1,000 were "killed or disappeared" while the UN mission gave an initial death toll of 330, saying that while both camps were involved in the mass killings, the majority of deaths were caused by pro-Ouattara fighters. Ouattara's government instead accused "the loyal forces, mercenaries and militias of Laurent Gbagbo" of being behind mass graves discovered in the area. The aid organisation Doctors Without Borders on Sunday voiced alarm over the number of wounded people flooding into its medical centres in the west, saying medical supplies and medicine were running out and that the fighting was hampering access to the injured. "There is real concern for some patients, who can practically no longer be treated in Ivory Coast," it said in a statement. The French military meanwhile said 167 foreigners, including French and Lebanese nationals, left Abidjan Sunday for the Senegalese capital Dakar on a special flight.