French and UN helicopters have fired on military camps operated by Ivory Coast incumbent leader Laurent Gbagbo, in an effort to halt attacks on civilians.
The presidential palace was also hit in the helicopter attacks, witnesses said.
The strikes came as fighters backing Mr Gbagbo's rival Alassane Ouattara stepped up their attempts to take control of the main city, Abidjan.
Mr Gbagbo has refused to quit despite UN-approved results saying Mr Ouattara won an election last November.
'The house was shaking'
In recent days, pro-Ouattara forces have launched a sustained assault, sweeping down from their northern strongholds to capture the rest of the country.
Witnesses say they met little resistance until reaching the main city of Abidjan, a Gbagbo stronghold.
For five days, the city has been engulfed in fighting.
Pro-Ouattara commanders say they have now launched their final assault on the city and expect to take it in a matter of days.
Many residents in Abidjan, a city of five million people, are said to be trapped indoors without food, water or electricity.
Pro-Gbagbo forces have been accused of launching attacks on civilians and UN staff.
The UN said an attack helicopter had fired missiles at heavy weapons in the Akouedo military camp.
An Abidjan resident told the BBC's Focus on Africa programme that the military base had been under attack for an hour.
"There was a helicopter noise, we heard a bombing, and the house was shaking," the resident said.
France's military fired on another base, the Agban camp, saying the UN had asked for urgent help to take out heavy weaponry and protect civilians.
Paris-based Gbagbo spokesman Toussaint Alain said the helicopter strikes were "illegal, illegitimate and unacceptable".
"France has gone to war against Ivory Coast," he said.
The UN, which has 9,000 peacekeepers in the country, said it had the mandate to respond to heavy weapons attacks against UN staff or civilians.
UN chief Ban Ki-moon insisted rebuffed the idea that the UN had taken sides in the conflict.
"In line with its Security Council mandate, the mission has taken this action in self-defence and to protect civilians," he said.
The BBC's Barbara Plett at the UN in New York says the action marks an escalation for the UN mission.
She says the mission has come under increasing pressure from the Security Council to take more aggressive action amid attacks on civilians and the mission's own staff.
Former colonial ruler France, which still has about 12,000 citizens in the country, had earlier taken control of Abidjan airport from the UN mission.
Meanwhile, the UN has sent an envoy to investigate a massacre of hundreds of civilians in the western town of Duekoue last week.
Both sides have said the other was responsible for the killings.
The UN said on Saturday that more than 330 people had been killed - mostly by Mr Ouattara's forces.
However, the Caritas aid agency estimated that 1,000 may have been killed.