"Over the course of these years we have seen too many incidents of radiological materials finding their way from one path to another, outside the normal control mechanisms of nations," Admiral James Loy, deputy secretary of the US Department of Homeland Security told reporters.

Asked about the possibility of an attack with chemical, biological or a nuclear dirty bomb built from radioactive material and conventional explosives, Loy said: "Our concern for (such a) likelihood is raising all the time."

"In the wrong hands weapons of mass destruction have the capacity of raising the stakes dramatically," he said on the sidelines of a security conference in Vienna of the pan-European Organization for Security and Cooperation in Europe (OSCE).

EU counter-terrorism coordinator Gijs de Vries said however that "the general assessment is that conventional attacks are more likely at the moment."

"But we must be prepared" for weapons of mass destruction attacks, he said.

Both anti-terrorism exports said so-called biometric passports, which use readings from eye retinas or fingerprints, should be put into use as soon as possible to avoid false use of such identity documents.

Loy said two other areas where the 55 nations of the OSCE should focus their efforts in the coming year were better monitoring of containers transported by ship and international exchanges of information in order to crack down on stolen passports.

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