The United States has again warned its nationals of the likelihood of an Islamic insurgency attack in the Middle East.

The warning came in wake of an Al Qaida attack in Saudi Arabia that killed five Western engineers and executives. Two of the casualties were U.S. nationals, Middle East Newsline reported.

The U.S. embassy in Riyad has urged Americans to leave the kingdom. A warden message by the embassy was issued hours after the Al Qaida attack on the Yanbu petrochemical and refinery complex on May 1.

"Private American citizens currently in Saudi Arabia are strongly urged to depart," the message said. The State Department updated its warning to U.S. citizens of what it termed the continuing threat of anti-American violence and terrorist attacks against U.S. citizens and interests, specifically in the Middle East, including the Red Sea, Persian Gulf, the Arabian Peninsula and North Africa. The update warned of anti-American protests as well as insurgency attacks on a range of transportation assets in the region.

"Credible information has indicated terrorist groups may be planning attacks against U.S. interests in the Middle East," the State Department update said. "Terrorist actions may include suicide operations, bombings, hijackings or kidnappings. These attacks may involve aviation, ground transportation and maritime interests."

The State Department said conventional weapons such as explosive devices comprise a more immediate threat in many areas of the Middle East. But the warning did not rule out the use of biological or chemical weapons by insurgency groups.

The warning reported increased security at official U.S. facilities throughout the region. As a result, the department said, Al Qaida-aligned insurgents have sought to target so-called soft targets such as public transportation, residential areas, and areas where Westerners congregate.

The State Department said travel of official personnel at embassies and consulates around the world would be restricted because of security concerns. Staffers were urged not to leave their homes or offices other than for official business and medical appointments. They said private U.S. citizens should avoid the same areas restricted to government personnel.