"Today, I authorized the armed forces of the United States to begin a limited military action in Libya," said Obama.

Pentagon officials said US and British warships and submarines fired 110 Tomahawk cruise missiles at Libya's air defence systems in "Operation Dawn," the first phase of military action against Libya to impose a UN-mandated no-fly zone.

"We must be clear: actions have consequences, and the writ of the international community must be enforced," Obama told reporters while on an official visit to Brazil.

Qadhafi had defiantly warned France, Britain and the United States that they will regret interfering in his country's affairs, following a UN resolution that allowed the use of force to protect civilians from advancing pro-Qadhafi forces.

The US president stressed, however, that the operation would not expand into US boots on the ground in Libya.

"As I said yesterday, we will not, I repeat, we will not deploy any US troops on the ground," Obama said.

The Pentagon said the barrage of Tomahawk cruise missiles struck more than 20 targets, including surface-to-air sites, early warning sites, and communications facilities.

The first missile struck at 1900 GMT following air strikes carried out earlier by French warplanes.

"I want the American people to know that the use of force is not our first choice, and it's not a choice that I make lightly," Obama said.

"But we cannot stand idly by when a tyrant tells his people that there will be no mercy, and when his forces step up their assault on cities like Benghazi and Misrata where innocent men and women face brutality and death at the hands of their own government."