WASHINGTON - The Obama administration said Thursday that Moammar Gadhafi's government may be targeting Libyan civilians with cluster bombs, cautiously endorsing claims by rebels and human rights groups that the Libyan strongman's troops are using the indiscriminate weapon on the western city of Misrata.

Attacks by Gadhafi's forces have been deplorable, Secretary of State Hillary Rodham Clinton said. Despite outlining more examples of what she termed Gadhafi's "inhumanity," Clinton refused to signal any new course for the United States to help anti-government forces in their war to end four decades of dictatorship.

"Col. Gadhafi's troops continue their vicious attacks, including the siege of Misrata," Clinton told reporters in Washington. "There are even reports that Gadhafi forces may have used cluster bombs against their own people."

She also offered her condolences to the friends and families of two Western journalists killed in Misrata on Wednesday: Tim Hetherington, 40, a British-born war photographer and the Oscar-nominated co-director of the documentary "Restrepo," about U.S. soldiers on an outpost in Afghanistan; and Chris Hondros, 41, a New York-based photographer for Getty Images.

Two other photographers -- Guy Martin, a Briton affiliated with the Panos photo agency, and Michael Christopher Brown -- were treated for shrapnel wounds, doctors said.

Pressing the case of journalists in Libya, Clinton demanded the immediate release of all American citizens "unjustly detained," including at least two reporters. James Foley, of the Boston-based GlobalPost, and Clare Morgana Gillis, a contributor to The Atlantic and USA Today, were taken prisoner on April 5 by forces loyal to Gadhafi.

"I say 'at least' because we do not have any accurate information coming from Libyan authorities about other inquiries that we have made regarding their continuing harassment and detention of journalists, including Americans," Clinton said.

The situation in Misrata, Libya's third-largest city, is dire after almost two months of siege by Gadhafi's troops. Hospitals are unable to cope with casualties, including many shrapnel injuries. Hundreds have been killed in relentless attacks, residents say.

Rebels in Misrata have alleged that Gadhafi's forces are using cluster bombs, which pose particular risk to civilians because they scatter small bomblets over a wide area. And New York-based Human Rights Watch backed up the claim last week after it said its researchers inspected remnants and interviewed witnesses. But the Obama administration hadn't addressed the claim before Thursday.

Libyan officials have persistently denied the army is shelling Misrata or using cluster bombs. "We welcome any objective investigation of the actions of our army, our government and our officials," said government spokesman Moussa Ibrahim. The international community shouldn't "listen to media reports or stories fabricated by the rebels," he said.

Cluster bomblets can be as small as flashlight batteries and are packed into artillery shells or bombs dropped from aircraft. A single container used to destroy airfields or tanks and soldiers typically scatters hundreds of the mini-explosives over an area the size of a football field.

The U.S. used the weapon, a descendant of the "butterfly bomb" dropped by Nazi Germany on Britain in World War II, in Southeast Asia during the Vietnam War and in battlefield situations in Iraq and Afghanistan. The bombs also were used by Soviet and Russian troops in Angola, Afghanistan and Chechnya, where leftover duds also continue to inflict casualties, particularly on children attracted by their often eye-catching colour and little parachutes.

They most recently were used by both sides in the war between Russia and Georgia three years ago, according to human rights groups.

The campaign against the weapons picked up steam after Israel's month-long war against Hezbollah in 2006, when it scattered up to 4 million of the munitions across Lebanon, according to the U.N. In response, more than 100 countries pledged to ban the bombs. The United States has rejected the call, insisting that the bombs are a valid weapon of war when used properly. Libya also has never signed a treaty banning them.

Usually 10 to 15 per cent -- but in some cases up to 80 per cent -- of the devices fail to explode immediately. Those that don't detonate right away may do so later at the slightest disturbance.

One cluster bomb last week exploded just hundreds of yards from Misrata's main hospital, according to a report cited by U.N. human rights chief Navi Pillay. She said it was inevitable that weapons such as cluster munitions, multiple rocket launchers and other heavy weaponry would lead to civilian casualties if used in crowded urban areas. Government attacks will be scrutinized by the International Criminal Court, she warned.