WASHINGTON - The United States has pressed for Libyan leader Muammar Gadhafi to step down, but a leadership vacuum raises concerns about the security of Libya's weapons stockpiles and the danger of them falling into the hands of adversaries, officials said on Monday.

Libyan rebels have taken over most of Tripoli, Gadhafi's location is unknown, and great uncertainty exists about who will eventually end up in charge of the country.

House Intelligence Committee Chairman Mike Rogers warned of security concerns while Gadhafi's rule crumbles.

The Organization for the Prohibition of Chemical Weapons in February said Libya kept 9.5 tonnes of mustard gas in a secret desert location guarded by the army, but had destroyed aerial bombs designed to deliver chemicals in 2004 as part of a short-lived rapprochement with the West.

Gadhafi's stockpiles of chemical agents are still being closely guarded by forces loyal to the Libyan leader, a US official said on Monday.

The United States, NATO and the United Nations have been keeping a close eye on the stockpiles during the crisis, officials said.

"The stockpiles at this point appear to be well-guarded," the US official said, speaking on condition of anonymity. "It's worth keeping in mind that Gadhafi did in fact destroy many of his most dangerous weapons, and that much of what remains is outdated or difficult to make operational."

A UN official told Reuters that due to their age, Libya's chemical stockpiles might be more of an environmental hazard than a military or terrorist threat.

They consist of "very old chemical components which are not very useful as weapons," the official said. Mustard gas decays with age and Gadhafi's stockpiles are old enough that they are not even necessarily that hazardous, the UN official said.

US and European officials also are concerned about keeping secure Libya's stockpiles of conventional weapons - surface-to-air missiles, anti-tank rockets, armored vehicles, rocket-propelled grenades and explosives.

Libyan forces fired three Scud-type missiles on Monday from the area of Sirte, Gadhafi's home town. That followed the launch of another Scud missile last week, the first time his forces fired the weapon since the conflict began.

Some counter-terrorism officials were much more concerned about Gadhafi's arsenals of conventional weapons being looted than they were about his stockpile of chemical agents, a European security official said.

The fear is that such weapons could either make their way to militant groups or insurgents seeking to destabilize other African governments. But so far there was little evidence of significant weapons leaks or militant involvement in Libyan forces opposed to Gadhafi, a US official said.

"As we move forward, the international community must ensure a peaceful transition where the will of the Libyan people is heard," US Representative C.A. "Dutch" Ruppersberger said.

"We must also ensure radical extremist groups do not take control of the country. Libya has a large stockpile of chemical weapons and explosives that must not fall into the wrong hands," said Ruppersberger, the senior Democrat on the House Intelligence Committee.