The U.S. Embassy in Saudi Arabia has warned that a terrorist group may be planning to abduct Westerners in the capital, Riyadh, and it has urged American citizens to exercise caution.

In a message posted on its website Wednesday, the embassy advised U.S. citizens to carry out regular security measures, including varying personal travel routines and minimizing their public profile. It gave no further details about the possible plot.

Saudi Arabia has waged a determined crackdown on Islamist militants since al-Qaida's Saudi branch launched a wave of attacks in the country in 2003, including suicide bombings and shootings that killed dozens of Saudis and foreigners. At least 11 Americans were among the dead.

The kingdom's security forces largely crushed the militants by 2006, arresting or killing most of its known leaders. But the government fears al-Qaida could use its base in neighboring Yemen to restart operations in Saudi Arabia.

Saudi authorities also are concerned that Iran could provoke dissent among minority Shi'ites to destabilize the kingdom, home to Islam's holiest sites.