The 19 NATO members, in a message of solidarity with the US, have agreed to invoke the collective defence clause of the North Atlantic Treaty, if Washington determines that Tuesday's terrorist attacks were masterminded from abroad.
The decision opens the way for US allies in Europe to support any potential US military action against those responsible.
NATO Secretary General Lord Robertson said: "What the allies are saying is what they did was unacceptable, barbaric, and we stand today in solidarity with the US," he said.
US Secretary of State Colin Powell said once invoked, it would make it easier, for instance, for the US to obtain permission from allies to allow US warplanes fly through their airspace. Lord Robertson said he believed it was the first time in NATO's 52-year history that the alliance has positioned itself to put its principle of collective defence into action.
The European Union declared tomorrow a day of mourning for the victims the attacks and promised to spare no efforts to bring the perpetrators to justice.
EU foreign ministers, meeting in emergency session in Brussels, issued a declaration condemning the attacks promising Washington all possible help with search and rescue operations. "The Union will spare no efforts to help identify, bring to justice and punish those responsible: there will be no safe haven for terrorists and their sponsors," the ministers said.
In an unusual move, Lord Robertson joined the foreign ministers for yesterday's meeting. His presence, he said, underlined the common purpose of the two organisations in response to Tuesday's events.
The Minister for Foreign Affairs, Mr Cowen, praised the work of a 40-strong team of Irish diplomats who have dealt with thousands of calls from worried relatives since the attacks. He insisted that, under international law, the US had the right to self-defence following the attacks and rejected a suggestion by the Green MEP Ms Patricia Mckenna that Washington's response should be non-violent.
"Rather than anticipating a response, I want it known unambiguously that we should stand with the US at this time. We share their values," Mr Cowen said.
Ms McKenna made the comments while speaking in an emergency debate at the European Parliament. "If the US responds with attacks on possible suspect countries, they too will be killing more innocent people just like those who died so tragically yesterday and they will also be encouraging further counter-attacks. Two wrongs never make a right," she said.
