Belgian police Friday arrested 14 Muslim extremists suspected of planning to free an Al Qaeda sympathizer imprisoned for planning a terrorist attack on U.S. air base personnel, officials said.

Security was heightened at airports, subway stations and other public places across the capital, and the U.S. Embassy issued a warning to Americans that "there is currently a heightened risk of terrorist attack in Brussels" although it had no indication of any American targets.

Belgian Prime Minister Guy Verhofstadt said the government had information suggesting "preparation of an attack."

"Other acts of violence are not to be excluded," he said.

The prisoner, Nizar Trabelsi, a 37-year-old Tunisian who played soccer for several German teams, was sentenced to the maximum 10 years in prison four years ago. He had admitted planning to drive a car bomb into the canteen at Kleine Brogel, a Belgian air base where about 100 American military personnel are stationed.

According to the U.S.-based military affairs think tank GlobalSecurity.org, the Kleine Brogel Air Base is home to Belgian F-16 warplanes equipped with nuclear weapons that are under U.S. control.

Trabelsi, who testified that he intended kill American soldiers, says he met Usama bin Laden in Afghanistan and asked to become a homicide bomber. He was arrested in Brussels on Sept. 13, 2001. Police later discovered the raw materials for a huge bomb in the back of a Brussels restaurant.

The federal prosecutor's office said the 14 were planning to free the terrorist by force. "Trabelsi would be helped by a group of people, driven by an extremist vision of Islam," said a statement from the prosecutor's office.

The Interior Ministry called on citizens to be vigilant throughout the Christmas period. "You can point out possible suspect objects and actions to the local police," said a statement.

The U.S. Embassy issued its alert to U.S. citizens living or traveling in Belgium to maintain a high level of vigilance, especially in crowded public places,

Its statement said it had "no information to indicate that U.S. citizens or facilities are an intended target."