Governments are expecting more terror attacks as more ISIL jihadists leave Syria as air strikes continue against their positions.
The US State Department has taken the rare stop of alerting all travellers and expats to the danger of kidnapping, bombings and shootings worldwide.
Officials warn of a continuing threat from terrorist attacks at sporting events, theatres, markets and on public transport.
“Foreign governments continue to raid terrorist hide-outs and disrupt plots of violence,” said a spokesman.
“The US is routinely sharing intelligence with other governments to defend against terror threats.
“But travellers should expect travel disruption, extra security screening and delays at short notice.”
Read the US State Department full alert
Stricter border controls in Europe
New border controls are in operation between Denmark and Germany and Sweden and Norway. Travellers will need passports to cross between countries
The German government has ordered extra security at Christmas events and public buildings.
Several police and security guards were killed when terrorists blew up a bomb on a bus in the city of Tunis.
The government has declared a national state of emergency that will last until Christmas Eve.
The measure includes a 9pm to 5am curfew every day.
Airlines suspend Sharm el Sheikh flights
EasyJet and British Airways have suspended flights to the Egyptian Red Sea resort of Sharm el Sheikh until the New Year.
The action follows the downing of a Russian passenger jet killing all 224 people on board by a terrorist bomb.
ISIL has claimed responsibility.
Charter airline Monarch has no flights to the resort until December 19, while Thomson and Thomas Cook intend to start flights on December 9.
The Foreign Office has advised against all but essential travel to or from Sharm el-Sheikh, explained no UK airlines are flying in or out of the resort.
The US government has also warned against travel to North Korea due to the risk of arrest and long term detention stemming from inconsistent application of criminal laws in the country.