The United States issued a worldwide travel alert Friday, warning Americans to be vigilant during the holiday period, especially in the wake of a deadly siege in Australia.

Sydney this week saw a dramatic siege in a cafe that began Monday and left two hostages and a gunman dead.

"The lone wolf attack in Sydney, Australia... resulting in the deaths of two hostages, is a reminder that US citizens should be extra cautious, maintain a very high level of vigilance and take appropriate steps to enhance their personal security," the State Department alert read.

The agency said an analysis of past attacks "strongly suggests" terrorists this holiday season are focusing not only on US government facilities but also on hotels, shopping areas, places of worship and schools.

"US citizens abroad should be mindful that terrorist groups and those inspired by them can pose unpredictable threats in public venues," the alert said.