The CIA is secretly using an airbase in southern Pakistan to launch the Predator drones that observe and attack al-Qaeda and Taleban militants on the Pakistani side of the border with Afghanistan, a Times investigation has found.

The Pakistani Government has also repeatedly demanded that the US halt drone attacks.(Veronique De Viguerie) The Pakistani and US governments have repeatedly denied that Washington is running military operations, covert or otherwise, on Pakistani territory - a hugely sensitive issue in the predominantly Muslim country.

The Pakistani Government has also repeatedly demanded that the US halt drone attacks on northern tribal areas that it says have caused hundreds of civilian casualties and fuelled anti-American sentiment.

But The Times has discovered that the CIA has been using the Shamsi airfield - originally built by Arab sheikhs for falconry expeditions in the southwestern province of Baluchistan - for at least a year. The strip, which is about 30 miles from the Afghan border, allows US forces to launch a Drone within minutes of receiving actionable intelligence as well as allowing them to attack targets further afield.

It was known that US special forces used Shamsi during the invasion of Afghanistan in 2001, but the Pakistani Government declared publicly in 2006 that the Americans had left it and two other airbases.

Key to the Times investigation is the unexplained delivery of 730,000 gallons of F34 aviation fuel to Shamsi. Details were found on the website of the Pentagon's fuel procurement agency.

The Defence Energy Support Centre site shows that a civilian company, Nordic Camp Supply (NCS), was contracted to deliver the fuel, worth $3.2 million, from Pakistan Refineries near Karachi.

It also shows the fuel was delivered last year, when the United States escalated drone attacks on Pakistan's lawless tribal areas, allegedly killing several top Taleban and al-Qaeda targets, but also many civilians.

A source at NCS, which is based in Denmark, confirmed that the company had been awarded the contract and had supplied the fuel to Shamsi, but declined to give further details.

A spokesman for the US embassy in Pakistan told The Times: "Shamsi is not the final destination." However, he declined to elaborate and denied that the US was using it as a base.

"No. No. No. No. No. We unequivocally and emphatically can tell you that there is no basing of US troops in Pakistan," he said. "There is no basing of US Air Force, Navy, Marines, Army, none, on the record and emphatically. I want that to be very clear. And that is the answer any way you want to put it. There is no base here, no troops billeted. We do not operate here."

He said that he could not comment on CIA operations.

The CIA declined to comment, as did the Pentagon. But one senior Western source familiar with US operations in Pakistan and Afghanistan told The Times that the CIA "runs Predator flights routinely" from Shamsi.

"We can see the planes flying from the base," said Safar Khan, a local journalist. "The area around the base is a high-security zone and no one is allowed there."

He said that the outer perimeter of Shamsi was guarded by Pakistani military, but the airfield itself was under the control of American forces.

Shamsi lies in a sparsely populated area about 190 miles southwest of the city of Quetta, which US intelligence officials believe is used as a staging post by senior Taleban leaders, including Mullah Omar. It is also 100 miles south of the border with Afghanistan's southern province of Helmand and about 100 miles east of the border with Iran.

That would put the Predators, which have a range of more than 2,000 miles and can fly for 29 hours, within reach of militants in Baluchistan, southern Afghanistan and in Pakistan's northern tribal areas.

Paul Smyth, head of operational studies at the Royal United Services Institute, said that 730,000 gallons of F34, also known as JP8, was not enough to supply regular Hercules tanker flights but was sufficient to sustain drones or helicopters.

Other experts said that Shamsi's airstrip was too short for most aircraft, but was big enough for Predators and ideally located as there were few civilians in the surrounding area to witness the drones coming and going.

Farhatullah Babar, a spokesman for the President of Pakistan, Asif Ali Zardari, said that he did not know anything about the airfield. HOwever, Major General Athar Abbas, the chief military spokesman, confirmed that US forces were using Shamsi. "The airfield is being used only for logistics," he said, without elaborating.

He added that the Americans were also using another airbase near Jacobabad, 300 miles northeast of Karachi, for logistics and military operations.

Pakistan gave America permission to use Shamsi, Jacobabad and two other bases - Pasni and Dalbadin - for the invasion of Afghanistan in October 2001. US Marine Special Forces were based at Shamsi and, in January 2002, a US Marine KC130 tanker aircraft crashed close to its runway, killing seven Marines on board.

Jacobabad became the main US airbase until Bagram, near Kabul, was repaired, while Pasni, on the coast, was used for helicopters and Dalbadin as a refueling post for special forces' helicopters. However, in December 2001, Pakistan began sharing Jacobabad and Pasni with US forces as India and Pakistan began massing troops on their border. In July 2006 the Pakistani Government declared that America was no longer using Shamsi, Pasni and Jacobabad, although they were at its disposal in an emergency.

The subject has become particularly sensitive in the past few weeks as President Obama has made it clear that he will continue the strikes while reviewing overall US strategy in the region.

The latest strike on Monday - the fourth since Mr Obama took office - killed 31 people in the tribal agency of Kurram, and another on Saturday killed 25 people in South Waziristan, according to Pakistani officials.

Shah Mehmood Qureshi, the Pakistani Foreign Minister, responded on Sunday by categorically denying that Pakistani bases were used for US drone attacks.

Aerial assault

- Armed predator unmanned aerial vehicles (UAVs) have been in use since 1999

- The aircraft is controlled from the ground using satellite systems and onboard cameras

- The MQ9 craft, which is used in Afghanistan, is 11m long, has a 20m wing span and a cruise speed of up to 230mph. Each can carry four Hellfire missiles and two bombs

- Three systems were bought by the RAF last year for £500m

Sources: Jane's Information, US Airforce, RAF, Times archive

Reporting by Tom Coghlan in Kabul, Zahid Hussain in Islamabad and Jeremy Page in Delhi