The prosecutor's emphasis that France was under direct threat of attacks from Islamic State militants raised tension as the country went into a weekend of political campaigning and a vote on Sunday for a centre-right candidate for next year's presidential election.

Authorities said earlier this week that attacks had been planned for Dec. 1 against police and intelligence headquarters in Paris as well as the Disneyland Paris theme park.

Five suspects, arrested in France on suspicion of planning attacks on security targets in and around the French capital, were directed remotely by Islamic State in Iraq and Syria, the Paris prosecutor said on Friday.

France is already under a state of emergency, with soldiers patrolling the streets of the capital, following bomb and shooting attacks by Islamic militants in November 2015 which killed 130 people.

"An imminent attack was thwarted," Paris Prosecutor Francois Molins told a news conference.

"A Strasbourg commando team, and also a man arrested in Marseille, were given instructions to acquire arms. The instructions were given by a commander from the Iraqi-Syrian region via encrypted apps," he said.

Hand-written documents pledging allegiance to Islamic State and glorifying martyrdom were found at a house in Strasbourg, eastern France, Molins said.

Two of the five suspects had traveled to the Turkey-Syria border via Cyprus in March 2015, he said. Four of the men are French, while a fifth is Moroccan.

Police meanwhile continued searching for an armed man after finding a dead woman in a retirement home where some 60 missionaries were living near Montpellier in southwestern France, sources close to the investigation said on Friday.

One source said a caretaker had contacted the police after freeing herself after being bound and gagged by the suspect.