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 luggage and that one of them was carrying $7,000 in cash.

The circumstances of the security alert have prompted some security experts to suggest 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 terrorist attack.

Although suspicious items were found in the checked luggage of the two men, the items were not prohibited and were not considered dangerous and the men were therefore allowed to fly.

But U.S. media has reported that al Soofi, who had been booked on a flight to Washington Dulles International Airport, later boarded the Amsterdam-bound flight without his luggage instead, prompting U.S. authorities to alert Dutch authorities.

White House spokesman Robert Gibbs told CNN the men were involved in a series of suspicious events and that 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.

SECURITY NOT JEOPARDISED

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 that the matter was being taken very seriously. A lot of 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.

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.

(Reporting by Harro ten Wolde, Aaron Gray-Block and Ben Berkowitz in Amsterdam and Jeremy Pelofsky in Washington; Editing by Jon Hemming)