The US State Department appeared poised to issue a formal alert Sunday telling Americans traveling in Europe to remain vigilant of possible terror strikes, a US official said.
The alert -- which the State Department issues regarding specific events, and is one step down from a travel warning -- follows intelligence reports suggested an Al-Qaeda attack could be imminent.
"We are contemplating a travel alert for Europe," the US official told AFP on Saturday, speaking on condition of anonymity.
"The bottom line would be to tell Americans to continue to travel but be vigilant. The alert could be issued as early as tomorrow (Sunday)," the official said, without giving a specific reason.
Another US government official told AFP the State Department would take "further actions as appropriate."
"We have been and continue to be focused on Al-Qaeda's interest in attacking us, our allies and our interests. We will spare no effort to thwart terrorists' plans, and will take further actions as appropriate," the official said.
News media in the last week reported that western intelligence agencies had uncovered an Al-Qaeda plot to launch attacks in Britain, France, Germany and the United States.
The reports said well-armed, commando-style teams of jihadists planned to seize and murder Western hostages in a manner similar to the attacks two years ago in the Indian city of Mumbai on two hotels and its main railway station, in which 10 gunmen killed 166 people and injured more than 300.
Intelligence and diplomatic officials in Europe and the United States so far have refused to confirm the alleged terror plot on the record.
A US official privately confirmed the reports on Wednesday, but said it was not clear when and where the terror strike was meant to be launched.
"The threat is, at this point, credible but not specific," said the official, who asked to remain unnamed.
"It's unclear, for instance, precisely where something might occur. For that reason, people shouldn't limit their thinking to the United Kingdom, France, or Germany," the official told AFP.
"And while no one should dismiss the prospect of a Mumbai-style operation, it's entirely conceivable that other modes of attack are in play."
In Paris, one of the most popular European destinations for Americans, the Eiffel tower was evacuated twice last month after two bomb hoaxes.
The German weekly Der Spiegel reported that an Al-Qaeda plot to launch such attacks on European cities was planned by the group's number three leader, with support from Osama bin Laden.
In its issue to hit newsstands Monday, Der Spiegel reported that Al-Qaeda's third in command, Sheikh Yunis al-Mauretani, plotted the attacks, and had shared his plans with Ahmad Siddiqui, an Islamist with German nationality currently held at the US-run Bagram Air Base in Kabul, Afghanistan.
Siddiqui was the likely source of information that sparked the recent hikes in Western security threat levels, the weekly said, adding that German intelligence agents were expected to travel to Afghanistan soon to question him.
Travel alerts "are issued to disseminate information about short-term conditions, either transnational or within a particular country, that pose significant risks to the security of US citizens," the website said.
"Natural disasters, terrorist attacks, coups, anniversaries of terrorist events, election-related demonstrations or violence, and high-profile events such as international conferences or regional sports events are examples of conditions that might generate a Travel Alert."
Currently, a travel alert exists for India through November 15 due to the 2010 Commonwealth Games scheduled to be held in New Delhi between October 3 and October 14, it said.
A total of 31 travel warnings are in effect for various countries including Afghanistan, Iraq, Iran, Israel and Lebanon as well as the Democratic Republic of Congo, Sudan and Somalia.
