The State Department issued a world-wide caution against the potential for al-Qaida attacks on Western targets.
"European governments have taken action to guard against terrorist attack and some have spoken publicly about the heightened threat conditions," the warning read. "Extremists may elect to use conventional or non-conventional weapons and target both official and private interests."
The State Department said it was concerned about the potential for a terrorist attack in response to the May killing of al-Qaida leader Osama bin Laden.
In Yemen and North Africa, the department added, threats against Western targets are high. Al-Qaida in the Arabian Peninsula, Yemen's franchise, has targeted U.S. interests in the past and al-Qaida in the Islamic Maghreb has declared its intention to attack Western targets in east Africa.
Piracy remains a concern near the Horn of Africa and the State Department warned there was an increased threat of terrorism in India and Afghanistan.
U.S. citizens are warned that terrorists could attack soft targets like public transportation systems and tourist-related infrastructure.
The U.S. Department of Homeland Security didn't issue a corresponding alert.
"We remain at a heightened level of vigilance," the department noted.