Greece has announced that it has formally asked NATO for help in protecting this August's Athens Olympic Games.

"The government has asked for the contribution of NATO for the security of the Olympic Games," a Foreign Ministry statement said.

"This contribution will concern air control and joint patrolling of the sea and for protection against a nuclear, biological or chemical attack."

Greece is spending three times as much on security as Sydney did in 2000 because of increased terror fears since the September 11, 2001 attacks on U.S. cities.

A secret Olympics security operation involving several hundred American troops started on Wednesday, a day after Greece's new Prime Minister Costas Karamanlis took charge of the ministry responsible for Olympic preparations.

As part of its $790 million (440 million pound) security plan, Greece has set up a seven-nation advisory group, comprising Britain, the United States, Israel, Australia, Spain, France and Germany. Russia and Turkey are also helping.

International Olympic Committee president Jacques Rogge will meet Karamanlis in Athens on Saturday to discuss the new government's plans to speed up preparations.