The US State Department said it had "credible information" that the Islamic State (IS) and Al Qaeda were planning attacks and focusing on the "upcoming holiday season", reported the Telegraph.
It warned American travellers to exercise caution at "holiday festivals, events and outdoor markets".
The travel warning was issued as the US military said it expected IS to resort to more traditional terrorist attacks.
"We don't think they are going to become an organisation that no longer presents a danger once Raqqa and Mosul are taken away," said Col John Dorrian, the spokesman for the US-led coalition against IS.
"What they are going to do is devolve into the type of terror organisation that we've known they were all along and continue to try to do external operations and try to motivate lone wolf attackers."
The State Department warning was released a week after the one-year anniversary of the 2015 Paris attacks, which killed 130 people across the French capital.
It does not mention any specific countries as being particularly at risk but notes that 2015 saw attacks in Belgium, France, Germany, and Turkey.