US officials have been imposing stringent security measures as the anniversary of 11 September raises fears of another attack, both at home and abroad.

There have been security alerts on two passenger jets in the US, but most of the airports have been quieter than usual.

Some chose to fly - wearing patriotic colours

Air traffic overall was down by about a third compared with last week, according to the Federal Aviation Administration.

But with aviation security already a top priority, standards were tightened even more for the anniversary, with more air marshals on flights.

Security screeners seemed to outnumber passengers at some airports, as jittery Americans stayed away. To reassure fellow travellers, passengers flying into and out of New York and Washington-area airports were asked to remain seats for 30 minutes after take-off and 30 minutes before landing.

A security alert led an American Airlines jet bound for Dallas to return to Houston Intercontinental Airport shortly after take-off.

Two passengers have been taken into custody, although police say they did not appear to have been carrying any weapons.

One of the passengers had apparently been holding a folding comb that had been mistaken for a knife.

Earlier a Northwest Airlines jet was diverted from Las Vegas to Arkansas and four passengers were detained.

Cheney on standby

At US airports, all hand baggage was searched twice. Shoes with thick heels were checked with x-rays and all hats were taken off to be to make sure they were not hiding anything.

The country moved to its second-highest state of terrorism alert on Wednesday, as heat-seeking anti-aircraft missiles were deployed to guard the Pentagon and Washington and fighter jets patrolled the skies.

Skies over New York and Washington are under heavy guard

With ceremonies taking place to remember the more than 3,000 people who died in the 11 September attacks, US Vice-President Dick Cheney was taken to a secret, secure location so he could take over if anything happened to the president.

All American military forces in the Middle East, as well as troops in the Horn of Africa and parts of Central Asia, were put on their highest state of alert for the anniversary, a defence official told Reuters news agency.

Attorney General John Ashcroft warned on Tuesday that US intelligence had information that Osama Bin Laden's al-Qaeda network was planning more attacks, perhaps in South Asia or the Middle East. As a precaution he announced the temporary closure of several embassies and consulates in those regions and in Africa.

In the Gulf state of Bahrain, US troops actually went on "Threat Condition Delta" or its highest alert status, on Tuesday.

A message on the embassy's website said attacks against it were planned and that "such actions may be imminent and include suicide operations".

Mr Ashcroft also warned that national monuments at home could be a target.

Radioactivity

At Newark, New Jersey, across Upper New York Bay from Ground Zero, the US Coast Guard ordered a cargo vessel back out to sea after finding trace amounts of radioactivity on board.

The Liberian-registered ship was told to anchor 10 kilometres off shore, where the Coast Guard was conducting further inspections.

Attacks on two US embassies in Africa in 1998 killed 224 people

BBC security correspondent Frank Gardner said if any threats were to come from al-Qaeda on Wednesday it would be out of character, because the organisation does not usually pay much attention to anniversaries.

Attacking US interests on 11 September, our correspondent said, would make al-Qaeda seem a little more predictable - and make its operatives easier to catch.

But the US Government was not taking any chances.

The BBC's Jon Leyne in Washington said Mr Ashcroft's was one of the most specific and detailed warnings issued by the government in a year of high alerts.

President George Bush said he was taking indications of new threats against the US "seriously", and that their pattern was similar to what had been heard prior to 11 September 2001.