NATO Secretary-General George Robertson has raised the security alert status at the alliance's Brussels headquarters following reports of possible terrorist attacks in Europe.
"We are a military headquarters, a multinational headquarters with a lot of flags flying at the gate," Mr Robertson told said. "But we also have a lot of precautions in place and any group of terrorists will be ill-advised to try meddling with us."
He said he had raised the alert status two weeks ago at the sprawling complex on the edge of Brussels from Alpha Plus to Bravo, which officials said makes access more difficult.
Mr Robertson said there were "consistent reports that al Qaeda still plans further atrocities and that would fit into their pattern".
"The intelligence sources indicate there are cells and there are ambitions to do something in Europe. There are precautions in place but people should not be complacent."
On Friday, French daily Le Figaropublished an interview with the head of Interpol in which he said al Qaeda seemed to be preparing simultaneous attacks in several countries, including the United States, the dominant ally among NATO's 19 nations.