The United States has unleashed a barrage of Tomahawk missiles against the Libyan regime's air defenses, but ruled out using ground troops in what President Barack Obama called a "limited military action."

After taking a cautious stance on armed intervention in Libya's civil war, Obama ordered the strikes citing the threat posed to civilians by Muammar Gaddafi's forces and a UN-mandated no-fly zone endorsed by Arab countries.

"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 Saturday.

"We are answering the calls of a threatened people. And we are acting in the interests of the United States and the world," he said, stressing that Washington was acting in concert "with a broad coalition."

But with nearly 100,000 US troops fighting a protracted war in Afghanistan -- and with Saturday's missile strikes coming eight years to the day after the United States launched its war in Iraq -- Obama made clear that operation "Odyssey Dawn" would not send US troops to Libya.

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

In a dramatic show of force, US warships and a British submarine fired at least 110 Tomahawk cruise missiles into Libya against Gaddafi's anti-aircraft missiles and radar, the US military said.

Admiral William Gortney told reporters at the Pentagon that the cruise missiles "struck more than 20 integrated air defense systems and other air defense facilities ashore."

The first missile struck at 1900 GMT following air strikes carried out earlier by French warplanes, said Gortney, director of the US joint staff.

"It's a first phase of a multi-phase operation" to enforce the UN resolution and prevent the Libyan regime from using force "against its own people," he said.

The missile strikes came despite skepticism in the US military over the risks of intervention, with Defense Secretary Robert Gates repeatedly expressing caution.

Gates, who postponed a planned trip to Russia on Saturday for at least 24 hours, had suggested that Washington ought to think twice before going to war in another country in the Middle East.

But as rebel forces appeared on the verge of collapse and the Arab League came out in favor of a no-fly zone, the Obama administration opted to support joint military action first advocated by France and Britain.

The Pentagon has suggested the US military will play a supporting role in operations, employing Tomahawk missiles, electronic jamming aircraft and other resources while European allies fly bombing missions over Libya.

Gortney said the United States and its allies are not yet enforcing a no-fly zone with aircraft patrolling the skies round-the-clock, but "we're setting the conditions to be able to reach that state."

He declined to comment on whether the US military would send in fighter jets and bomber aircraft to carry out raids.

The targets included surface-to-air missile sites as well as early warning radar and command-and-control communication centers.

According to The Washington Post, Western reconnaissance satellites are closely monitoring a small garage at a remote desert site, south of the city of Sirte, where the Libyan government keeps about 10 tons of mustard gas.

Western officials are concerned Gaddafi could use the caustic chemical to kill large numbers of his people, the report said.
Gortney said it was too early to say how effective the night-time Tomahawk strikes were.

The US operation followed initial missions by French warplanes, which carried out four air strikes Saturday, destroying several armored vehicles from Gaddafi's forces.

The US missile attacks were carried out by two guided-missile destroyers, the USS Stout and the USS Barry, and three submarines in the Mediterranean near Libya, the USS Providence, the USS Scranton and USS Florida, the Navy said.

The Tomahawks have a range of 2,500 kilometers (1,500 miles) and carry a 450-kilogram (1,000-pound) warhead.