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The defense readiness condition, dubbed DEFCON, is an alert system used by the US military to indicate the risk of nuclear war.

The system has five levels of readiness, or states of alert, increasing in severity from DEFCON 5 – the least severe – to DEFCON 1 – the most severe.

The current DEFCON level is understood to be 5 – the lowest state of readiness.

But conspiracy theory website DEFCON warning system claims the threat has been upgraded to level 3, meaning the US Air Force is ready to mobilise in just 15 minutes.

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In a statement on its website, it claims that while there are no imminent nuclear threats against the US the situation is considered “fluid and can change rapidly”.

The website, which bases its warning on "current affairs", writes: “Tensions between Russia and the United States have reached levels beyond the cold war in the recent week.

“The situation between Russia and the United States is extremely fluid at the moment. In all likelihood as dynamic as at times during the Cuban Missile Crisis of 1962.

“At this time, the DEFCON Warning System feels that an increase to DEFCON 3 would be a prudent move.”

Singling out Syria as “trigger point” for increased conflict between the US and Russia, the website says the situation is very sensitive and “has the potential to spiral out of control”.

Tthe website stresses it is not affiliated with the US government or military and advises the public to “make their own evaluations”.

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The warning comes amid fears the next world war could kick off today as President Barack Obama meets top foreign policy advisors to discuss battling Russia.

The US leader is set to lay out plans for military options in Syria in a crunch meeting that could spell disaster for the world.

Meanwhile, a Ukranian helicopter was reportedly shot down by pro-Putin rebels with NATO officals on board, sparking fears of a western backlash.

A US military expert is also warning Alaska residents to prepare for a full-scale Russian invasion.