WASHINGTON -- Thousands of Americans were killed yesterday through "evil, despicable acts of terror," a sombre President George W. Bush told his country in a televised address last night.

"The pictures of airplanes flying into buildings, fires burning, huge structures collapsing have filled us with disbelief, terrible sadness and a quiet, unyielding anger," he said.

The President called his country's military "powerful and . . . prepared" as he all but declared war on states that harbour terrorists.

"We will make no distinction between the terrorists who committed these acts and those who harbour them," he said in a televised address to a nation still reeling from the worst terrorist acts in history.

Reflecting on the anger that has seized America after a day of infamy in which thousands -- perhaps tens of thousands -- of people died, Mr. Bush said the search is under way for the terrorists.

Unmistakable in the President's brief address was a clear determination to strike -- and strike hard -- against countries that assist or harbour known terrorists.

Chief among them is Afghanistan where Osama bin Laden -- the fugitive Saudi financier who has vowed to strike at the heart of America -- is known to be living.

But there are other countries, Algeria, Syria and Iraq among them, that may also soon face ultimatums: Deliver known or suspected terrorists or face punishing attacks from America's military.

For much of the day, Mr. Bush was out of sight in a bunker at an air force base in Nebraska -- far from the chaotic scenes of devastation, death and heroism in the financial and political hearts of the United States.

With no target to threaten and no one yet to blame, the U.S. government was all but invisible as a stunned nation watched endless television replays of the horrific terrorist attacks on New York's twin World Trade Towers and Washington's Pentagon.

But the President flew back to Washington late yesterday afternoon, with Air Force One escorted by F-16 and F-15 warplanes, to deliver a message of resolve to a still shaken nation.

"Our very freedom came under attack," he said. "A great people has been moved to defend a great nation."

However, the tough talk came after a day of stunning tragedy and awesome failures.

It was clear that there had been abject breakdowns in the sole remaining superpower's intelligence services and its military.

"No warnings," admitted Ari Fleischer, the President's spokesman, an admission that the most massive, sophisticated and successful terrorist strike in history had eluded America's octopus of intelligence agencies.

Meanwhile, there was no explanation of how four airliners could be hijacked and flown -- in at least two cases hundreds of kilometres and for nearly an hour -- without being successfully intercepted. That one ploughed into the Pentagon, supposedly protected by surface-to-air missiles, dramatically demonstrated U.S. vulnerability.

All three buildings were struck by suicide terrorists who hijacked airliners and turned them into massive missiles. The fourth airliner went down in western Pennsylvania.

Amidst a massive security clampdown, Mr. Bush was whisked aboard Air Force One and flown from Florida, where he was touting his education plans to a Sarasota kindergarten when the first jet slammed into the World Trade Center, to Shreveport, La.

There he made a brief statement that was televised nationwide. He was then taken to a secure bunker at Strategic Air Command at Offutt air force base in Nebraska.

House and Senate leaders were also in secret secure bunkers and Congress was suspended.

Vice-President Dick Cheney was in a command post in the White House.

Defence Secretary Donald Rumsfeld helped the wounded in the Pentagon and then spent the day in an underground command bunker.

In the perpetual struggle between the Secret Service -- which wants to hide the President -- and the political imperatives that he be visible as a symbol of resolve and leadership, protection won for most of the day.

He was described as "in continuous communication" with cabinet members and had talked by telephone with other world leaders. In the afternoon he chaired a national security video-conference.

"Parents need to reassure their children everywhere in our country that they're safe," first lady Laura Bush said as she was hustled from a Senate hearing and taken to a secure location in Washington.

America's mighty military -- impotent against four, lumbering, fuel-packed jetliners -- was placed on high alert around the world.

For the first time ever, U.S. warplanes patrolled the otherwise empty skies above New York and Washington.

Two aircraft carriers steamed out of Norfolk, Va., headed for New York.

But their mission was to support the rescue and recovery operation rather than retaliate.

"Needless to say, all elements of the United States government are now doing their part not only to help those who have been hurt, but to collect information, to analyze it, and to provide it to the President," Mr. Fleischer said aboard Air Force One.