The US State Department has issued a worldwide travel alert warning US citizens of the risks of travelling because of what it described as "increased terrorist threats".
"Current information suggests that (Islamic State), al-Qaeda, Boko Haram, and other terrorist groups continue to plan terrorist attacks in multiple regions," the department said in a warning posted on its website on Monday.
A State Department official noted that the agency has issued worldwide travel alerts in the past and said this latest alert effectively updated past warnings.
The department did not provide specifics of any planned attacks or threats, and mentions no specific regions, except to note attacks this year in France, Nigeria, Denmark, Turkey and Mali.
"US citizens should exercise vigilance when in public places or using transportation," it said, advising Americans to avoid large crowds or crowded places and to "exercise particular caution during the holiday season".
It also noted the threat of "lone wolf" attacks by unaffiliated persons inspired by terror groups, warning "the likelihood of terror attacks will continue as members of ISIL/Daesh return from Syria and Iraq".
It was referring to foreign fighters who return home after having fought alongside Islamic State extremists.
"Extremists have targeted large sporting events, theatres, open markets and aviation services," the alert said.
The alert expires on February 24.
