The United States has begun moving warships and aircraft closer to Libya and has frozen $30 billion in Libyan assets.

The White House is ramping up pressure on leader Muammar Gaddafi after calling on him to step down.

The ships could be used for humanitarian and rescue missions, Secretary of State Hillary Clinton said in Geneva, where she told the UN Human Rights Council that Gaddafi was using "mercenaries and thugs" to suppress a popular uprising.

"There is not any pending military action involving US naval vessels," she said after the Pentagon announced it was moving warships and air force units closer to Libya.

The Obama administration has said military action is one option it is looking at, although many analysts say the United States is highly unlikely to launch a ground invasion or air strikes because of the volatile situation on the ground.

The Pentagon gave no details of the forces being moved, but its announcement is likely aimed at sending a signal to Gaddafi and his government that the United States is matching its sharper rhetoric of recent days with action.

Other foreign governments are also increasing the pressure on Gaddafi to leave in the hope of ending fighting that has claimed at least 1,000 lives.