Troops supporting the UN-recognised president of Ivory Coast, Alassane Ouattara, appear poised for a final push to oust his rival, Laurent Gbagbo, who refuses to give up the presidency.
In the main city, Abidjan, pro-Ouattara forces have launched an assault on the fortified presidential residence.
French troops have taken about 500 foreigners to a military camp amid increasing insecurity in the city.
Mr Ouattara's supporters launched a fresh offensive on Monday.
The BBC's John James in the central city of Bouake says this looks like the final hours for Laurent Gbagbo's government.
An Abidjan resident said fighting was raging in the northern district of Cocody, where the presidential residence is located.
"I can still hear heavy gunfire and loud thud of mortar fire. And it is coming from the direction of Cocody," he told the BBC early on Friday.
Another resident told AFP news agency: "We are hearing deafening artillery shots, RPG7 [rockets] and machine guns."
Later, a plume of smoke was seen rising from the residence, which is protected by members of the elite presidential guard and located on a peninsula in Abidjan's lagoon.
Mr Gbagbo has not been seen in public for weeks. It is not clear whether he is at the compound.
A spokesman, Abdon Georges Bayeto, told the BBC: "The president is not going to step down. He's been elected for five years and we are going to put up a fight."
French forces say they have taken about 500 foreigners, including 150 French nationals, to a military camp after they were threatened by looters in Abidjan.
"There is a security vacuum and that has opened the way for looters to roam the streets," French military spokesman Thierry Burkhard told Reuters news agency.
A Swedish woman working for the UN in Abidjan was killed by gunfire at her home late on Thursday.
UN and French peacekeepers have taken control of Abidjan's international airport.
Mr Ouattara's government has said Ivory Coast's land, sea and air borders had been closed until further notice. It also declared that there would be a curfew from 2100 GMT to 0600 GMT in Abidjan until Sunday.
Mr Gbagbo has refused to relinquish the presidency since November's election.
But the national army has put up almost no resistance since the start of the offensive by Mr Ouattara's supporters, who are now believed to control about 80% of the country.
The United Nations human rights office urged Mr Ouattara to rein in his forces, citing allegations of abductions and attacks on civilians.
"We are receiving unconfirmed but worrying reports that [pro-Ouattara forces] have been committing human rights violations during their advance towards Abidjan," said Rupert Colville, a spokesman for the UN Commissioner for Human Rights.
Our correspondent says growing panic seems to be setting in among his supporters, especially following the decision of the head of the army, Gen Phillippe Mangou, to seek refuge with his wife and children at the home of the South African ambassador.
On Thursday evening, Mr Ouattara's TV channel featured senior military officers pledging allegiance to his government.
The head of the UN mission, Choi Young-jin, said as many as 50,000 soldiers, police and gendarmes had abandoned Mr Gbagbo, with only the Republican Guard and special forces personnel remaining loyal.
Mr Ouattara's government has said Mr Gbagbo will be made available to the International Criminal Court.
On Friday the African Union renewed its call on Mr Gbagbo to stand down.
Mr Ouattara was internationally recognised as president last year, after the electoral commission declared him winner of the November run-off vote.
The UN, which helped organise the vote, certified it as legitimate. However, Mr Gbagbo claimed victory after the Constitutional Council overturned Mr Ouattara's win.
The forces supporting Mr Ouattara have made lightning advances since Monday, moving out from their base in the north. On Wednesday, they captured the capital, Yamoussoukro, and the key port of San Pedro.
Sanctions and a halt to cocoa exports in what is the world's biggest producer of cocoa beans have brought West Africa's second-biggest economy to its knees, with banks closed for more than a month.
An armed rebellion in 2002 split the nation in two - a division the elections were meant to heal.
where we go?
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