(AP Photo)

The U.S. State Department today issued a worldwide travel alert, following Thursday's announcement that 21 embassies across the Muslim world, normally opened on Sunday, will be closed Aug. 4 because of a specific and credible terrorism threat.

The alert warns Americans that "terrorists may elect to use a variety of means and weapons and target both official and private interests," and reminds U.S. citizens of potential targets such as public transportation and tourist sites, and to take precautions.

So what should you do if you are an American living or traveling abroad?

• Enroll in STEP : The most important thing, U.S. officials say, is for travelers to enroll in the Smart Traveler Enrollment Program, otherwise known as STEP. There are 6 million Americans enrolled in the program who receive email messages and texts about travel alerts and warnings, both globally and country-specific. For example, an official told reporters that U.S. citizens living in or planning to visit Egypt can receive a message about protests in Tahrir Square, without having to go to the State Department website to look it up. There is also a STEP app that Americans can download to their smartphones.

• STAY AWARE: As the alert highlights, Americans should either avoid or be careful when visiting crowded tourist destinations and areas frequented by foreigners. Be aware of protests and the political situation in a country, for example upcoming elections that could turn violent.

• CALL THE EMBASSY: Can Americans in trouble still receive help when an embassy or consulate is closed? Yes, the official said. The closures affect routine public services like passport and visa issues, but "emergency services are still available. You can always call the [embassy] emergency number and receive service."