"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 siege two years ago of two Indian hotels in Mumbai, 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.
However, a US official on Wednesday privately confirmed the reports, but said it was not clear when and where the attack 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."
Travel alerts are one step down from travel warnings, and the State Department says alerts usually refer to specific short-term events, according to its website.
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 to November 15 due to the 2010 Commonwealth Games scheduled to be held in New Delhi between October 3 and October 14, it said.
State Department travel alerts are also in effect for Kenya and Bolivia and two have been issued due to Pacific typhoon season and the hurricane season in the Atlantic and Pacific.
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.
--------------------------------------------------------------------------