Libyan and NATO secret agents kept watch over hidden stocks of mustard gas, stockpiled by ex-dictator Muammar Gaddafi, throughout the war which toppled him to prevent his forces using them.

While one stockpile was known to the United Nations, the new Libyan regime on Tuesday revealed the existence of two other, formerly unknown, sites with stocks prepared by Gaddafi.

One contained weapons 'ready for immediate military use', according to Libyan expert Yussef Safi ad-Din, in charge of the problem of dealing with the gas which causes serious chemical burns in the eyes, on the skin, and in the lungs.

Gaddafi revealed one site, in the oasis of Al-Jafra near Waddan in central Libya, but had carefully hidden from the UN the existence of two others.

Safi ad-Din said both secret sites had now been 'securitised' and posed no health risk.

UN officials in 2004 had inspected the stockpile which Gaddafi acknowledged and its elimination had begun in 2010 under international control, continuing until February 2011, the start of the uprising against the dictator.

The gas remaining there - 11.25 tonnes - had been 'neutralised' by additives which drastically diminished its toxicity, said Safi ad-Din.

Fear of the gas being used by Gaddafi loyalists had preoccupied his opponents since the start of the conflict and a special team of local technicians and others from NATO had been set up, working secretly from Benghazi, the headquarters of the rebellion.

The cell 'was directed by General Abdel Fatah Yunes', chief of staff of the rebels and former interior minister in the Gaddafi regime. The general was killed on July 28 in mysterious circumstances, said Safi ad-Din, who was himself a member of the cell.

"The first stage was to maintain surveillance of the chemical arms which Gaddafi controlled, and prevent him from using them," the Libyan expert said.

That operation was a success, acknowledged Mansur Daou, interior security chief in the overthrown regime, imprisoned at Misrata, 215 kilometres (135 miles) east of Tripoli.

"Gaddafi had quickly abandoned the idea of using chemical weapons, the Americans were watching over them from too near. We could not get near them," without being bombed from the air, he said.

The Benghazi-based cell was also responsible for watching over the recovery of small quantities of radioactive material used by Libyan industry, notably cobalt 60 which could have been used to make a 'dirty' bomb, spreading radioactivity.

"The second step was to take control of all the chemical sites. Our forces conquered them one by one," recalled Safi ad-Din.

As a precaution, gasmasks were also distributed to a great many fighters, including more than 10,000 in the month of May at Misrata which just managed to beat back the Gaddafi forces after savage fighting and where the rumours of a threat from chemical weapons spread time and again.

Today, a team of American and Libyans, including Safi ad-Din, is based in Waddan to deal with the problems. The Americans, pistol on hips, scowled when journalists approached them, refusing any comment.

"It's the CIA (Central Intelligence Agency)," said a senior military officer from Misrata who had met them.

The watch over the mustard gas, at Waddan as well as the two other sites whose location is kept secret, is tight, according to the Libyan expert and local fighters.

"Three weeks ago, the car of two fighters was destroyed by an air strike as they got too near the bunkers containing the gas" at Waddan, without anybody even being hurt, laughed Ahmed Misrati, a fighter.

Stun bombs had also already been used against other fighters roaming on the site some days earlier, recalled Safi ad-Din who said that all the curious ones risk an air strike if they get closer than 50 metres (yards) to the chemical bunkers.

On Monday, the UN Security Council made its voice clear, adopting a resolution calling on Libya to end proliferation in the region of looted weapons that had been amassed by the Gaddafi regime.