Americans in the Middle East and North Africa were warned Thursday they may be targets of terrorists using biological weapons as well as conventional arms.

Seaborne vessels traveling in the southern Red Sea (search) are among the possible targets, the State Department said in alerting Americans to ongoing security concerns.

Also, the department said it was concerned extremists may be planning to carry out attacks against Westerners and oil workers on the Arabian Peninsula (search).

Americans considering seaborne travel near the Horn of Africa (search) in the southern Red Sea should exercise extreme caution, the department said.