But officials found themselves treading a fine line between caution and not causing panic. In a pre-released clip from a Christmas television interview due to be broadcast last night, President Bill Clinton said Americans should make the most of the holiday season but should immediately report "anything suspicious".
The assistant attorney general responsible for counter- terrorism measures, Eric Holder, had earlier advised Americans travelling abroad to avoid large gatherings, but reassured them it would be safe to attend large millennium events in the US where there would be "an extensive law enforcement presence". NBC television, however, cited "unconfirmed intelligence reports" that New York, Washington and Seattle had been selected for attacks at New Year celebrations. The network said the US authorities were working on information that an Islamic terrorist group operating out of Montreal may have planned attacks in as many as six US cities.
That theory appeared to originate with the arrest near Seattle last week of Ahmed Rossam, an Algerian citizen, who had arrived by ferry from Canada with a car boot full of bomb-making materials. A second Algerian, like Rossam also formerly resident in the Montreal area, was arrested trying to cross into the north-eastern US state of Vermont on forged identity papers.
The authorities have said they have no evidence that the two are linked or that either has a direct connection with the Saudi-born international terrorist, Osama bin Laden, who is suspected of masterminding last year's bombings of US embassies in Kenya and Tanzania.
Indeed, as the anti-terrorist alerts in the US proliferated this week, the ambassador to Pakistan from the ruling Afghan Taliban organisation, broadcast a formal assurance from the Taliban to the US that no attacks on Americans were planned over the New Year period. The US State Department described the assurance as "laughable", saying that the US had reliable information that Mr bin Laden was "in frequent contact with terrorists".
Meanwhile, all points of entry into the United States continued to be on a high state of alert, with hundreds of additional police, security officers and sniffer dogs deployed.