Witnesses and a UN official told news agencies that the Golf Hotel in Abidjan, Ivory Coast’s main city, had come under mortar and small-arms fire.
Alassane Outtara was judged to have won a presidential election in Ivory Coast, but Mr Gbagbo has refused to step down.
He has been surrounded in his residence for days by pro-Ouattara troops.
Reports suggested Saturday’s fighting flared at around 1700 GMT.
“The FDS (pro-Gbagbo Defence and Security Forces) are attacking us and we are trying to push them back,” one fighter with the pro-Ouattara forces told the AFP news agency.
“The firing is very very close. Snipers fired bursts from Kalashnikovs. The pro-Gbagbos are attacking us on all fronts,” a hotel resident added.
Mr Ouattara’s forces have swept down from the north of Ivory Coast over the past two weeks but much of Abidjan is dominated by Gbagbo supporters. Days of fighting have plunged the city into crisis.
It had appeared several days ago that Mr Gbagbo was on the verge of defeat but the upscale Plateau and Cocody areas of Abidjan are now fully in the control of his forces.
UN peacekeeping chief Alain Le Roy said late on Friday that Mr Gbagbo’s followers had in fact made strong gains, having used a pause of negotiations as an opportunity to regroup.
The UN has certified Mr Ouattara as the winner of November’s run-off vote for president but Mr Gbagbo has refused to cede power.
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