The U.S. Embassy in Yemen received a threat about a potential attack on its compound, an American official said Monday, triggering heightened security measures in the capital, San'a.

U.S. Embassy spokesman Ryan Gliha told The Associated Press that the embassy was urging American citizens to exercise caution in Yemen. He said the embassy received a threat "regarding a possible attack that could take place in the foreseeable future" but gave no further details.

In September, gunmen and two vehicles packed with explosives attacked the U.S. Embassy in San'a. Seventeen people were killed during the attack, including six militants. Al Qaeda later claimed responsibility.

A Yemeni security official said the U.S. Embassy received a telephone call and an e-mail early Monday saying the U.S. and Russian embassies in Yemen would be targeted by Al Qaeda within a few hours.

The official said the Russian mission didn't receive such a call but Yemen was taking the threat seriously. He didn't provide more details and spoke on condition of anonymity because he was not authorized to talk to the media.

Attempts to contact the Russian Embassy were not successful.

Police were seen setting up security checkpoints and turning back cars and pedestrians on two key streets heading toward the U.S. Embassy in eastern San'a. There were also checkpoints around the Russian Embassy in downtown San'a, but traffic was not disrupted.

Later Monday, the U.S. Embassy issued a warden message to Americans in Yemen advising them to be cautious and take "prudent security measures in all areas frequented by Westerners."

Yemen is the ancestral homeland of Usama bin Laden and has an active Al Qaeda presence, despite government efforts to destroy the terror network, which has purportedly set up training camps in the country.