French ground troops have entered the centre of the main Ivory Coast city of Abidjan for the fist time as incumbent leader Laurent Gbagbo refuses to quit.
Helicopter gunships are firing in the area around Mr Gbagbo's residence.
On Sunday, UN and French helicopter gunships began a new operation they said was aimed at destroying heavy weapons near Mr Gbagbo's residence.
Mr Gbagbo has been refusing to cede power to internationally-recognised President Alassane Ouattara.
Loud explosions can be heard echoing around the lagoons which surround the centre of the city, says the BBC's Mark Doyle in Abidjan.
On Monday morning, a large column of French armoured vehicles left the French military base in Abidjan.
A spokesman for Mr Ouattara has said his troops were involved as well, and that this was the largest joint attack since the conflict began.
The action will not stop until Mr Gbagbo steps down, the spokesman added.
French helicopter strikes have partially destroyed Mr Gbagbo's residence, one of his ministers said.