The State Department on Monday issued a worldwide alert three days ahead of Thanksgiving cautioning travelers of "increased terroristic threats" from ISIS, al-Qaeda, Boko Haram and other groups.
The alert expires Feb. 24.
Such warnings are typically not based on credible intelligence regarding a specific threat, but the alert says that Islamic militants are believed to be planning additional attacks in "multiple regions."
"These attacks may employ a wide variety of tactics, using conventional and non-conventional weapons and targeting both official and private interests," the alert says.
Hundreds — and possibly thousands — have been killed this year during terrorist attacks in France, Nigeria, Denmark, Turkey, Mali; ISIS has also claimed responsibility for bombing a Russian passenger plane.
Sports events, theaters and markets have all been targeted.
"Be aware of immediate surroundings and avoid large crowds or crowed places," said the alert, which didn't specifically mention the Friday Nov. 13 attacks in Paris. "Exercise particular caution during the holiday season and at holiday festivals or events."
France and Belgium have launched a manhunt following the attacks in Paris, with a focus on Brussels barkeeper Salah Abdeslam, 26, who allegedly returned to the city from Paris hours after the attacks and is still at large.
Fearing an imminent threat of a Paris-style attack, Belgium on Monday extended a maximum security alert in Brussels for a week but said the metro system and schools could reopen on Wednesday.
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"We are still confronted with the threat we were facing yesterday," Prime Minister Charles Michel said. Potential targets remained shopping areas and public transport.
"These attacks may employ a wide variety of tactics, using conventional and non-conventional weapons and targeting both official and private interests."
Belgium has been at the heart of investigations into the Paris attacks since French law enforcement bodies said two of the suicide bombers had lived there. Three people have been charged in Belgium with terrorist offences, including two who traveled back with Abdeslam from Brussels.
French jets from the Charles de Gaulle aircraft carrier struck Islamic State targets in Iraq and Syria on Monday, while Britain offered France the use of an air base in Cyprus to hit the militants behind the Paris attacks.
French President Francois Hollande met British Prime Minister David Cameron in Paris as part of efforts to rally support for the fight against Islamic State. Hollande is also due to visit Washington and Moscow this week. The French president and U.S. President Barack Obama will hold a joint news conference on Tuesday morning, the White House said.
Cameron offered air-to-air refueling services and said he was convinced Britain should carry out air strikes alongside France and would be recommending that Britain's parliament vote through such measures.
French jets taking off from the country's flagship in the eastern Mediterranean destroyed targets in Ramadi and Mosul in Iraq on Monday in support of Iraqi forces on the ground, the French armed forces said in a statement.
In the evening, a raid was carried out on Islamic State's Syrian stronghold of Raqqa, where the French armed forces said planes destroyed several facilities including a command center.