A police spokesman declined to disclose the nationalities of those arrested or the nature of the suspicions against them. “I can tell you 12 people have been arrested,” he said.

It was not clear if any of the suspects had been charged, although the spokesman said that they were being held overnight.

One American passenger on board the Northwest Airlines flight NW0042 said that she saw a number of passengers who appeared to be of South Asian origin being taken off the aircraft in handcuffs at Schiphol international airport in Amsterdam.

A US government official told the Associated Press news agency that crew members and air marshals saw the passengers attempting to use mobile phones and pass them between themselves as the flight was taking off. “It was behaviour that average passengers wouldn’t do,” the official said.

Another US official, quoted on CNN, said that some of the passengers pulled out mobile phones during take-off and passed them around. They also unbuckled seatbelts before the seatbelt signs were off. Air marshals had then broken cover and arrested them.

The aircraft had reached German airspace when the pilot asked air traffic control for permission to return to Schiphol airport and requested an escort of fighter jets.

Two fighters scrambled to accompany the DC10, which was carrying 149 passengers, to the airport.

“Some of them behaved in a suspicious manner in the opinion of the crew,” the Dutch Ministry of Defence said in a statement. Several passengers were taken off the plane for questioning after it landed safely, Pamela Kuypers, a spokeswoman for the airport, said. Others were questioned at the gate.

Airports around the world imposed tighter security checks two weeks ago after authorities in Britain foiled an alleged plot to blow up aircraft bound for the United States over the Atlantic using liquid explosives disguised as drinks.

But checks have been relaxed since then, and Ms Kuypers said that threat levels at Schiphol had returned to normal. The Netherlands’ security alert level has been at “substantial” — the second-highest in a four-stage warning system — since the July 7 bombings in London last year.

Schiphol is Europe’s third-largest cargo airport and fourth-largest passenger hub.

There have been several international air traffic alerts in recent days. On Friday a British aircraft made an emergency landing in southern Italy after a bomb scare, and the US Air Force scrambled jets to escort a United Airlines flight from London to Washington as it was diverted to Boston.

In Germany security fears have been heightened after a failed attempt to bomb two regional trains with makeshift devices last month. They failed to explode and one of the alleged bombers, a 21-year-old Lebanese student, was arrested on Saturday.

He had been filmed by a railway surveillance camera getting on a train with a second suspect, who is believed to have fled the country. Police are investigating whether they have links with al-Qaeda.

ON-BOARD ALERTS

August 16 The British students Sohail Ashraf and Khurram Zeb, both 22, are removed from a flight from Málaga to Manchester after fellow passengers complained they were acting suspiciously, checking their watches and regularly speaking Arabic

August 17 US fighter jets scrambled to escort a United Airlines flight from London to Washington after it was diverted to Boston, below, following a terror scare

August 18 Excel Airways aeroplane en route to Egypt diverted to Italy after a passenger found a note saying: “There’s a bomb on this aircraft”

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