Dutch authorities have detained two Yemeni men at Amsterdam's Schiphol airport on suspicion of terrorist conspiracy, though a U.S. official on Tuesday cast doubt that they were part of a terrorism plot.

The pair, flying from the United States to Yemen, were detained on Monday after U.S. authorities told their Dutch counterparts that they had suspicious items in their luggage, sparking a 24-hour trans-Atlantic scare.

U.S. authorities scrambled to examine the background of the two men, Ahmed Mohamed Nasser al Soofi and Hezam al Murisi, and found they did not know each other and were not likely testing U.S. aviation security, according to the U.S. official.

The United States has elevated airline security in the wake of the September 11, 2001 attacks and an attempt by a Nigerian man who tried to blow up a U.S. airliner with a bomb hidden in his underwear which was given to him by an affiliate in Yemen.

The Dutch prosecution service plans to decide on bringing charges in the next few days. No charges in the United States are expected, the U.S. official said.

The men are being held in custody on suspicion of conspiracy to commit a terrorist criminal act, the Dutch prosecution service said in a statement. A spokeswoman for the service declined comment after the U.S. official cast doubt on a plot.

The two men landed 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 (DHS) told Dutch authorities of the suspicious items in their luggage.

These included mobile phones found taped together and one phone taped to a plastic bottle, and one of the men had $7,000 in cash. The phones were seized in the United States. ABC News cited officials as saying three large knives were also found.

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

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 could have been conducting a test run for a planned attack. U.S. authorities stressed that their security procedures worked.

MISSED THEIR FLIGHT

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 in Chicago and were rebooked on the same flight to Amsterdam, according to another 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.

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, criticized 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.

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

Terrorism