Military police are to guard valuable cargo at Amsterdam's Schiphol Airport after thieves hijacked a security truck in what may have been the country's biggest ever diamond raid, the Dutch government said on Tuesday.

The announcement came as a military police spokesman said the number of detectives investigating Friday's robbery, in which Dutch media said jewels worth around EUR75 million (USD$99.10 million) were stolen, had been doubled to 40.

The largest ever gem theft recorded by Guinness World Records resulted in an estimated loss of USD$100 million when most of the vaults at the Antwerp Diamond Centre in Belgium were emptied.

Dutch Justice Minister Piet Hein Donner announced the extra temporary security measures in parliament, and said they included permanent surveillance on cargo ramps and armed escorts for valuable goods.

He said Schiphol Airport would try to bring forward the introduction of passes for staff including biometric data such as iris-scans to this year instead of 2006, and that employee screening by the intelligence service would be more intensive.

Industry organization Aircargo Nederland (ACN) said the robbery came as no surprise given that military police have been told to concentrate on fighting drug smuggling, human trafficking and preventing attacks.

"The transport of valuable goods is competing for supervision and control with priorities set by politicians and the marechaussee (military police)," Ben Radstaak of the ACN was quoted as saying in daily newspaper Het Financieele Dagblad.

Michel Einhorn of Cool Diamonds, who said he lost USD$1.2 million worth of diamonds in the theft, told Dutch daily Algemeen Dagblad he did not expect them to be found again and criticized security at the airport.

"Experience shows that if gems are not found within 24 hours, they are never found," he was quoted as saying.

"An airport is supposed to be amongst the most impenetrable places of the land. It is unthinkable that armed men entered the terrain and then left without a shot being fired. And I cannot even pass security with a nail clipper," he added.

KLM security staff were threatened during the robbery on Friday morning. No-one was injured.

The security truck belonging to KLM Cargo was later found abandoned but the thieves, who a source close to the investigation said were heavily armed, remain at large.

Officials declined to put a figure to the value of the stolen goods which were on their way to a flight bound for world diamond center Antwerp.

"We don't know the exact contents yet. Declarations from owners are still coming in," military police spokesman Rob Stenacker said.

Airline KLM-Air France said it would not give further details while the investigation by Dutch police was continuing.

In September, South African police foiled an attempt by five gunmen to rob KLM of valuable cargo at Johannesburg Airport.

Last May, British police arrested seven men at Heathrow Airport after foiling an attempted gold heist.