A global travel alert has been issued by the US state department, in light of an unspecified Al-Qaeda threat.
The department warned the potential for an attack was particularly strong in the Middle East and North Africa.
Earlier the US announced two dozen embassies and consulates will close tomorrow.
The alert warned of a potential attack on public transport or tourist infrastructure.
The US state department said the alert will expire on August 31st.
US President Barack Obama has called for "all appropriate steps" to be taken to protect American citizens.