World War III is trending on Twitter after a Russian military jet was shot down

Reuters

US soldiers march up a hill after the D-Day landings in 1944

World War III is trending on Twitter after a Russian warplane was shot down by Turkey on Tuesday.

An Su-24 came down after being fired upon by a Turkish F-16 fighter jet, Reuters reports.

CNN Turkey is reporting that one of the pilots is dead.

Almost immediately after the incident, WWIII began trending in the UK as people began speculating over how Russian President Vladimir Putin may respond.

It comes as the US State Department issued a worldwide travel alert due to "increased terrorist threats."

The threats are related to Islamic State as well as other jihadist terrorist groups such as al-Qaeda and Boko Haram.

The State Department said on Monday: "Authorities believe the likelihood of terror attacks will continue as members of ISIL/Da'esh return from Syria and Iraq. Additionally, there is a continuing threat from unaffiliated persons planning attacks inspired by major terrorist organisations but conducted on an individual basis. Extremists have targeted large sporting events, theatres, open markets, and aviation services."

Here's what it looks like on Twitter: 

Talk of a new global conflict has grown since Russia began military operations in Syria in support of Bashar al-Assad, Syria's embattled president.

Tensions have also been rising between Turkey, a NATO member, and Russia over the close proximity to which Russian planes have been operating to the Turkish border.

Under Article 5 of the North Atlantic treaty, member states are required to come to the aid of any fellow member state subject to an armed attack.

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Vladimir Putin has a close relationship with Syrian President Bashar al-Assad

In October, following news Turkey had shot down a drone which violated its airspace, Prime Minister Ahmet Davutoglu warned the same fate would happen to a plane.

"Whoever violates our borders, we will give them the necessary answer," Reuters reported him as saying at a rally of his ruling AK Party.