Checks were tightened on all vehicles and people entering and leaving the SHAPE (Supreme Heaquarters Allied Powers Europe) base near Mons in southern Belgium after the warning last Friday, said the official.

The state of alert was downgraded on Monday, he said, adding that an Interpol tip-off from Trieste, Italy, said that four terrorists were planning to attack the headquarters. "It is all over now," he told AFP.

Belgium's RTL television reported that the four alleged Islamic terrorists were from Greece, adding that the attack was planned for Sunday. The NATO official could not confirm this.

The North Atlantic Treaty Organization, whose main base is in Brussels, has been listed among threatened targets by Islamic militants warning of al-Qaeda attacks in Europe.

The alliance last year took over command of a peacekeeping force in Afghanistan, which it agreed to expand further outside Kabul expand last month at a meeting which also saw NATO leaders agreed to train Iraqi security forces.

WAR.WIRE