The United States State Department has issued a worldwide travel alert for all US citizens in the wake of the attacks in Paris, Mali and elsewhere in recent weeks and the ongoing threat from militants including Isis (also known as Islamic State). Boko Haram, al Qaeda and other groups are also said to be a danger. The alert says Isis militants returning from Syria may pose particular problems and also warns of the dangers of terrorists using "non-conventional" weapons - though it does not state whether it means biological or chemical weapons.

Paris Saint-Denis
Police operation in Saint-Denis, near Paris on November 18, 2015, five days after Paris terror attacks. THOMAS SAMSON/AFP/Getty Images

The notice says: "The State Department alerts U.S. citizens to possible risks of travel due to increased terrorist threats. Current information suggests that ISIL (aka Da'esh), al-Qa'ida, Boko Haram, and other terrorist groups continue to plan terrorist 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. This Travel Alert expires on February 24, 2016."

The warning also advises tourists to exercise caution at sports events, festivals and other major events. US citizens are advised to follow social media and follow the instructions of local authorities.

The State Department warning comes as Brussels remains in virtual lockdown due to fears of another terrorist outrage like the one which claimed 130 lives in Paris on 13th November. 22 people were killed in the attack on the Radisson Blu hotel in Mali last week and 224 people - mostly Russian holidaymakers - when Metrojet Flight 9268 was blown from the sky over the northern Sinai on 31 October. All have been blamed on Isis or Isis-inspired militants, as well as other less-publicised attacks elsewhere in the world.

The gloomy warning attempts to end on a reassuring note: "Foreign governments have taken action to guard against terrorist attacks, and some have made official declarations regarding heightened threat conditions. Authorities continue to conduct raids and disrupt terror plots. We continue to work closely with our allies on the threat from international terrorism."