Helicopter gunships have begun a new operation to destroy heavy weapons near Laurent Gbagbo's residence in Ivory Coast's main city Abidjan.
UN and French helicopters were attacking near the presidential residence and palace as well as military bases, a UN spokesman said.
A BBC correspondent reports loud explosions coming from the area.
A presidential spokesman accused France of seeking to kill Mr Gbagbo, who refuses to accept defeat at the polls.
He is believed to be sheltering in a bunker at the presidential residence after talks on a cease-fire with advancing forces loyal to his political rival Alassane Ouattara collapsed.
UN Secretary General Ban Ki-moon said in a statement he had requested the air strikes because Mr Gbagbo's forces were using heavy weapons against both the civilian population and his peacekeepers.
On Saturday, pro-Gbagbo forces attacked a hotel used by Mr Ouattara, who is recognised by the UN as the victor in November's presidential election.
Eight helicopters
At least six French helicopters and two UN helicopters roared into the sky from their station at the French military base in the south of the city within minutes of the UN decision to attack, the BBC's Mark Doyle reports from Abidjan.
Eyewitnesses then saw the helicopters attacking targets around the residence of Mr Gbagbo.
Our correspondent, reporting about 5km (three miles) from the presidential residence, could himself hear loud explosions coming from the area.
Smoke was seen rising from the area.
Our correspondent received one report that the residence had been partly destroyed and was on fire but he said he could not confirm this.
UN spokesman Hamadoun Toure told the BBC: "We sent a mission to try and locate all heavy weapons.
"We found found heavy weapons within the presidential palace area, also in the centre of the presidential residence, and in most of the camps in Abidjan. The operation is just to neutralise those heavy weapons."
In his statement, Mr Ban said attacks by Mr Gbagbo's forces on civilians and UN peacekeepers had to stop, and he had instructed UN forces to use "all necessary means" to prevent the use of heavy weapons.
He also called on Mr Gbagbo to resign immediately.
'Pretexts'
Responding to Sunday's attacks, Gbagbo spokesman Ahoua Don Mello said France, the former colonial power, was set on killing the incumbent president.
"There is no other goal [but] to assassinate the head of state, all the rest is just pretext," he told AFP news agency.
"France was looking for pretexts to attack again, and has found them."
On Tuesday, Mr Gbagbo was said to be "hours away" from surrender, but then he took refuge in a bunker under the residence.
The UN said on Friday his forces had used a lull in the fighting as a "trick" to regain ground in Abidjan and strengthen their position.
Last week, Mr Ouattara's forces swept down largely unopposed from their strongholds in the north. However, fighting began when they reached Abidjan, a city of four million people and the country's commercial capital.
