Dutch authorities detained two Yemeni men at Amsterdam's Schiphol airport on suspicion of terrorist conspiracy after a tip-off from the United States, prosecutors said on Tuesday.

The pair, en route from Chicago to Yemen, were detained on Monday and a decision on bringing charges is to be made in the next few days, the Dutch prosecution service said.

The men are being held in custody on suspicion of conspiracy to commit a terrorist criminal act, the prosecution service said in a statement.

The Yemenis arrived in Amsterdam from Chicago O'Hare airport on United flight 908 early on Monday and were detained on the plane after the U.S. Department of Homeland Security alerted Dutch authorities to suspicious items found in their luggage in the United States.

These included mobile phones found taped together, and one phone taped to a plastic bottle. The phones were seized in the United States. ABC News cited officials as saying three large knives were found in the men's checked luggage and that one of them was carrying $7,000 in cash.

Although suspicious items were found in the luggage, the items were not prohibited nor considered dangerous. They were therefore allowed to continue their travel.

This matter is under investigation but as of right now, these two passengers have not been charged with any crime in the United States and we caution you against jumping to any conclusions, DHS said in a statement.

U.S. President Barack Obama was briefed about the incident, according to the White House.

The circumstances of the security alert have prompted some security experts to question whether the men -- identified by a U.S. law enforcement official as Ahmed Mohamed Nasser al Soofi and Hezam al Murisi -- could have been conducting a test run for a planned attack.

U.S. authorities boosted security after the September 11, 2001 attacks when four airliners were hijacked and crashed, and again after a Nigerian man tried to detonate a bomb aboard a plane from Amsterdam to Detroit on Christmas Day last year.

Al Soofi flew from Birmingham, Alabama to Chicago while al Murisi started in Memphis, Tennessee. They were supposed to transfer to a flight to Washington Dulles International Airport and from there on to Dubai, a U.S. official said.

However, they missed the plane and were rebooked on the flight to Amsterdam, according to one U.S. official. Al Soofi's luggage made it to Dulles but was pulled from the Dubai-bound flight before it took off, the official said.


White House spokesman Robert Gibbs told CNN the men were involved in a series of suspicious events and authorities would conduct a vigorous investigation.

They were not on any of the terror watch lists ... They went through some extra screening. Their bags were pulled off of a flight because they were not on that flight, Gibbs said.

Extra precautions were taken ... and now obviously the next step is getting some answers.

As far as we know security has not been jeopardised, a spokeswoman with the Dutch counter-terrorism agency NCTb said.

She said Dutch authorities were in contact with the United States and the matter was being taken very seriously. Many details still needed to be clarified, she said.

Edwin Bakker, head of the security and conflict programme of the Netherlands Institute of International Relations, said the combination of factors, including the luggage and the money, was very suspicious and could indicate it had been a test run.

If this is a dry run, it is always bad news ... but it's good to see that counter-terrorism measures appear to work, he said. This looks like a very controlled operation and that authorities were very alert with a lot of co-operation.

Klaas-Arjen Krikke, a lawyer representing one of the men, criticised the information being released about the incident.

My client has already been condemned by a large section of the public via the media, Krikke told agency ANP.

An acquaintance of al Soofi said that he had worked at a gas station for the last year in Alabama and was not religious.

He just quit (his job) last Thursday or Friday, I believe, and said he was going to visit his brother in Tennessee and then going home to Yemen, the acquaintance, Ali Moqbel, told Reuters in a telephone interview, adding that Soofi had made prior trips to Yemen.

Passenger footage aired by Dutch commercial broadcaster RTL showed police escorting the handcuffed men out of the plane as passengers watched from their seats.

The arrests revived memories of an alleged attempted bombing last Christmas on a flight from Amsterdam to Detroit.

Suspect Umar Farouk Abdulmutallab is charged with trying to blow up a Delta Air Lines flight as it approached Detroit with an explosive device hidden in his underwear.

(Additional reporting by Harro ten Wolde and Ben Berkowitz in Amsterdam and Peggy Gargis in Birmingham; Editing by Jon Hemming and Jerry Norton)