As Interior Ministry officials announced the discovery of a suspected terrorist hideout containing large quantities of chemicals used to make explosives, a warning was issued for Americans in Saudi Arabia that a series of terrorist attacks may be in the pipeline, a spokesman at the U.S. Embassy in Riyadh (search) told FOX News on Wednesday.

The information originated from the American National Counter-Terrorism Center (search), the new hub for threat information in the United States.

"We urge people to be attentive," said State Department Deputy Spokesman Adam Ereli, who complimented the "closeness and aggressiveness" of the combined U.S.-Saudi efforts to go after militants in the kingdom.

Meanwhile, officials said they uncovered a reported militant lair near Riyadh in which they found bomb-making materials including fertilizer, ammonium nitrate and other chemicals.

The threat information was vague, with no indication of when, where and how attacks would be carried out, but was considered credible enough to issue a warning.

"Exercise caution and maintain good situational awareness when visiting commercial establishments frequented by Westerners or in primarily Western environments," the warning reads in part.

The U.S. Embassy in Riyadh has no plans to close, but the warning referred to previous attacks in "housing compounds and other establishments where Westerners may be located."

"We have issued a warden message about our knowledge of ongoing operation planning for terrorist attacks in the kingdom and we have urged American citizens to maintain a high level of vigilance," U.S. Embassy spokeswoman Carol Kalin told The Associated Press over the telephone.

In 1996, a truck bomb destroyed an apartment building in the Khobar Towers (search) military housing complex near Dhahran, Saudi Arabia, killing 20 people, mostly U.S. service members, and wounding 372. The U.S. government has blamed Lebanese Hezbollah (search) for the attack.

Since May 2003, Islamic militants in Saudi Arabia have carried out numerous bombings and kidnappings and have regularly battled security forces.

The attacks, which have tended to target Westerners, have been blamed on the Al Qaeda terror group and allied militants.

A U.S. official said the new information was credible enough to put embassy staff on high alert.

The Associated Press contributed to this report.