The United States has called on Laurent Gbagbo to step down as president immediately, saying he is pushing the West African nation of Ivory Coast into lawlessness.
US Secretary of State Hillary Clinton said he "must leave now so the conflict may end."
The appeal came as heavy artillery fire and explosions shook central Abidjan on the third day of a fierce battle for the city.
So far, Mr Gbagbo has been brushing off calls by world leaders to step down, amid an offensive by troops backing the internationally recognised president Alassane Ouattara.
Millions of civilians are trapped in Abidjan with food running out and refugees are arriving in neighbouring countries like Liberia.
The International Red Cross said that in what appeared to be inter-ethnic violence, at least 800 people were killed in an incident in the western town of Duekoue.
The BBC reports that French troops have taken over the airport in Abidjan.
The French military says an extra 300 French soldiers are being sent to Abidjan.
More than 1500 foreign nationals are under the protection of French troops in an army camp near the airport.