European Union leaders tonight backed "targeted" US retaliation against countries harbouring terrorists and said they were prepared to help.
"We reaffirmed our full solidarity not only from the heart but also from our sense of reason," French President Jacques Chirac said.
"To be clear, we will not sit on the sidelines of this battle against this scourge."
With flights over the city of Brussels banned and barbed wire sealing off the summit centre, the 15 government leaders met on three-days notice to demonstrate solidarity with the US at the highest level and coordinate their anti-terrorism efforts.
They backed the introduction of a European arrest warrant that would prevent lengthy extradition procedures currently in use, as well as a common definition of terrorism.
"The European arrest warrant will allow the direct handover of wanted people from one legal authority to another," the statement said.
They also called on judicial authorities around the EU to draw up a single list of terror suspects and organisations operating around Europe.
Within the Europol police unit, a special anti-terrorist team will be set up as soon as possible.
"This team will collaborate closely with its American counterparts," the final declaration said.
Additional reporting from AP