A special forces battalion trained to defend against weapons of mass destruction has begun deployment in Greece for the forthcoming Olympic Games and Paralympic Games, NATO said in a statement on Thursday.

Greece also has assurances that the alliance will assist in air and sea patrols and will command several U.S. special forces units that will be on standby during the Athens Games in case of a terrorist attack.

The weapons of mass destruction defence operation, known as Distinguished Games, is the first major mission of NATO's new Multinational Chemical Biological Radiological Nuclear Task Force. The battalion expects to have a task force of troops, vehicles and equipment from Belgium, the Czech Republic, Hungary, Italy, Poland and Spain at its base in the northern Greek beach resort of Halkida by August 1.

The team is not a fighting unit, but a specialised group that helps protect civilians and public and military installations against weapons of mass destruction or leaks from chemical factories and refineries.

NATO did not disclose the battalion's size.

It will carry out surveillance and detection to protect the August 13-29 Olympics and September 17-28 Paralympics. If necessary, it will also oversee decontamination operations.

NATO ally Greece has drawn up the most expensive security plan in Olympic history to guard the first summer Olympics since the September 11, 2001 attacks on the United States.

The estimated cost of 1.0 billion euros ($1.2 billion) is more than three times the amount spent to protect the Sydney 2000 Games.