The State Department said in a global travel warning Tuesday that Americans should take precaution and maintain vigilance about terrorist threats, demonstrations and the possibility of violence against U.S. citizens.
The department said attacks may be in the form of suicide operations, assassinations, kidnappings, hijackings and bombings.
Americans should consider the potential for attacks on transportation systems and tourist infrastructure, it said. It noted such attacks in Moscow, London, Madrid, Glasgow and New York in recent years.
The department also warned Americans to avoid demonstrations in Arab countries because they can turn violent.