US president reverses previously cautious approach to Libyan conflict and signs America up to more muscular intervention

President Obama today signals the return of America to the forefront of the international effort in Libya, writing a joint article with David Cameron and Nicolas Sarkozy in which the three leaders commit their countries to pursue military action until Colonel Gaddafi has been removed.

In the joint article, Obama reverses America's earlier cautious approach to the conflict – which saw the US hand control to Nato and withdraw fighter planes just days after the intervention began – and signs up his country to the more muscular intervention of his European colleagues.

Obama's new interest could transform the efforts of the international community after three days of talks in the Gulf state of Qatar in effect came to nothing.

Writing in Washington Post, the Times and Le Figaro (in French), the three leaders say the world would have committed an "unconscionable betrayal" if the Libyan leader is left in place, putting rebels who have been fighting against the Gaddafi regime at the mercy of his government. If left, Libya risks becoming a failed state, they write.

Obama, Sarkozy and Cameron call on Gaddafi to "go and go for good", rejecting demands for an immediate ceasefire and a negotiated exit for the Libyan dictator.

Diplomats are considering how the language of the United Nations mandate can accommodate a more active role on the ground.

In the piece, the three leaders acknowledge the legal constrictions but suggest they must overcome them: "Our duty and our mandate under UN Security Council Resolution 1973 is to protect civilians, and we are doing that. It is not to remove Gaddafi by force. But it is impossible to imagine a future for Libya with Gaddafi in power. The international criminal court is rightly investigating the crimes committed against civilians and the grievous violations of international law. It is unthinkable that someone who has tried to massacre his own people can play a part in their future government."

Yesterday Gaddafi was seen driving through Tripoli on what seemed to be a victory parade. His soldiers bombarded the city of Misrata with shells.

In their article the three leaders call the attack on Misrata a "medieval siege … to strangle its population into submission".

They write: "The brave citizens of those towns that have held out against forces that have been mercilessly targeting them would face a fearful vengeance if the world accepted [Gaddafi staying]. It would be an unconscionable betrayal.

"So long as Gaddafi is in power, Nato and its coalition partners must maintain their operations so that civilians remain protected and the pressure on the regime builds … Britain, France and the United States will not rest until the UN security council resolutions have been implemented and the Libyan people can choose their own future."

The Times reported on its publication that the article originally began as a collaboration between Cameron and Sarkozy, coinciding with the prime minister's trip to Paris on Wednesday to discuss the military action.

A draft was sent to the White House as a courtesy, prompting a request from Obama to add his name. Diplomatic sources said that only minor changes were made to accommodate him.