WASHINGTON (AFP) - President Barack Obama rallied to the support of France on Friday after the bloody end to hostage sieges in Paris, vowing to offer all assistance to combat the threat of militant Islamists.

"I want the people of France to know that the United States stands with you today, stands with you tomorrow," Obama said at a speaking engagement in Tennessee, describing France as America's "oldest ally".

"The moment that the outrageous attack took place, we directed all of our law enforcement and counterintelligence operations to provide whatever support that our ally needs in confronting this challenge," Obama added.

Obama was speaking hours after the crisis that began with the massacre of 12 people at satirical weekly Charlie Hebdo reached a dramatic climax, with French police storming two hostage sites, killing two brothers wanted for the newspaper attack and a jihadist ally.

"We're hopeful the immediate threat is now resolved. The French government continues to face the threat of terrorism and has to remain vigilant. The situation is fluid," Obama added.

"President Hollande made it clear they'll do whatever is necessary to protect their people and I think it's important for us to understand France is our oldest ally."

US Secretary of State John Kerry honoured the victims and congratulated French security forces Friday evening at the French embassy in Washington, where he signed a book of condolences.

Kerry offered his sympathies for the "nightmare" France has experienced in the past days.

He said the United States would "never allow the nihilism of an act of terror to deter us for one instant from our love for and commitment to freedom."

"Vive la France," French-speaking Kerry wrote in conclusion.