A hotel used by the internationally recognised president of Ivory Coast has come under attack by forces loyal to his rival Laurent Gbagbo, reports say.
Witnesses and a UN official say the Golf Hotel in Abidjan had come under mortar and small-arms fire, and the nearby UK embassy has been evacuated.
Alassane Ouattara was judged to have won the presidential election, 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.
AFP also reported that UN peacekeepers - tasked by the Security Council with protecting civilians in Ivory Coast - had fired back.
"The Golf Hotel was attacked with mortars," UN spokesman Hamadoun Toure said, adding that the attack had come from south of Abidjan's lagoon, away from the presidential residence.
"In conformity with our mandate to protect the Golf Hotel where President Ouattara and his team are, the peacekeepers responded by targeting the origin of the firing coming from the other side of the lagoon. We intentionally avoided the residence of President Gbagbo."
However, a spokesman for Mr Gbagbo denied that forces supporting him had carried out the attack.
"It's absolutely false. There has been no attack on the Golf (hotel). It's an imaginary attack." Ahoua Don Mello told AFP.
The decision to abandon the British embassy compound was taken after bullets hit the building and a mortar round landed in the garden.
The embassy is near the residence of Laurent Gbagbo.
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.
The BBC's Mark Doyle, in Abidjan, says an estimated one million people have been made homeless by the recent fighting, and there are growing concerns for public health.
"The situation is tragic in certain neighbourhoods" of Abidjan, Carlos Geha, of the UN Office for the Co-ordination of Humanitarian Affairs (Ocha) told AFP.
"It is not medicine or material aid that is missing but the means to get them to those who need them."
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 for negotiations as an opportunity to regroup.
Earlier this week Mr Ouattara's troops were reported to have isolated Mr Gbagbo in an underground bunker, but a pause in the fighting appears to have given his forces new resolve.
"They clearly used the lull of Tuesday as a trick to reinforce their position," Mr Le Roy said, referring to a dip in the fighting after three of Mr Gbagbo's generals requested talks.
Mr Le Roy said that there was evidence that his forces were still using weapons including tanks, M-21 rocket launchers, rocket-propelled grenade launchers and armoured personnel carriers.
Heavy weapons fire from Mr Gbagbo's forces targeted the French ambassador's residence in the city on Friday, the French embassy said, although this was denied by Mr Gbagbo's supporters.
On Saturday the remaining British presence in Abidjan left the city. A skeleton staff of two diplomats and 16 local staff were evacuated in a convoy of UN armoured vehicles.
The decision to evacuate was taken after bullets started coming in through the window and mortars landed in the garden, our correspondent says.
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.
Both men have been criticised for the actions of troops loyal to them during the crisis. Most recently Mr Ouattara's forces were criticised by Human Right Watch, which alleged they killed or raped hundreds of people in villages as they advanced on Abidjan.