A Middle Eastern group or nation is behind a threatened terrorist attack on American citizens living in Beijing that has prompted an unprecedented security alert, an embassy spokesman said Friday.

Chinese officials, who warned the U.S. Embassy of the threatened attack Wednesday, have given no more specific information on the source of the threat, said embassy spokesman Andrew Koss.Koss confirmed a report out of Washington that the threat had come from a Middle Eastern nation or organization.

Beijing has relations with most countries and organizations regarded as potentially hostile to the United States, including Iran, the Palestine Liberation Organization and Libya.

The U.S. Embassy issued the emergency security warning Thursday and conducted a phone and mail campaign to warn American citizens that "there is information that nationals of a third country may be planning a terrorist attack on Americans in Beijing."

Koss said the embassy has been flooded with calls from American citizens since the warning was issued. He said most callers were "concerned but not panicked."

The nature of the threat, which has sent shock waves through the capital's small American community, was not fully clear.

But U.S. officials were advised by a high-ranking representative of the Chinese government that it stemmed from a group of foreigners, a senior embassy security officer said.

It was the first such warning directed only at Americans in the Chinese capital, although China has been included in previous warnings by the U.S State Department.

Officials have advised U.S. citizens to take precautions similar to those observed in the Middle East and Latin America, such as varying travel routes, increasing observation and avoiding restaurants and hotels where Americans often congregate.

About 1,100 Americans are registered as residing in Beijing, but several hundred more, mostly students and teachers, live in the city, officials said. Up to 2,000 American tourists also are in Beijing at any one time, they said.

Foreign diplomats and journalists live in compounds guarded by Chinese police, but business representatives generally live in less secure hotels. Foreign teachers, students and "experts" working with Chinese companies and universities usually live in special dormitories.