A US Air Force 'sniffer' plane which took off from Sussex today was on a mission to find evidence of nuclear activity or explosion, according to strong rumours.

The WC-135 Constant Phoenix, which is specially modified to collect atmospheric samples, flew out of RAF Mildenhall on operational sorties.

The specialist equipment enables the crew to detect radioactive debris 'clouds' in real time is believed to be heading towards northern Europe and the Barents Sea.

News of the deployment comes amid claims Russia may be testing nuclear weapons, either to the east or in the arctic, after a spike in radioactivity was reported.

According to spotters a second 'spy' plane was also deployed from Mildenhall.

The WC-135 Constant Phoenix, which is known as a nuclear 'sniffer' plane, was deployed to Britain last week on an undisclosed mission (file image from a previous mission in 2011)

Air quality stations in Norway, Finland, Poland, Czech Republic, Germany, France and Spain have detected the presence of Iodine-131 at low levels

It is not the first time the Constant Phoenix has visited the British airbase, but the latest deployment reflects growing concern about an alleged spike in iodine levels recorded in northern Europe.

This has fuelled speculation that the WC-135 has been called in to investigate the cause of the higher-than-normal levels of Iodine-131.

Air quality stations across the continent detected traces of radioactive Iodine-131 in January and February, which seem to have come from eastern Europe.

The high levels of Iodine-131 has led some to suggest Putin is testing nuclear weapons in Novaya Zemlya near the Arctic.

The spike in Iodine-131 has sparked speculation that Russian president Vladimir Putin is testing nuclear weapons in Novaya Zemlya near the Arctic

However, the CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organisation) ruled out a nuclear test had recently taken place.

Similar aircraft were used in the wake of the Chernobyl nuclear plant disaster in the Soviet Union in 1986 and the Fukushima incident in Japan six years ago by collecting particles and chemical substances in the atmosphere, days, weeks and months after they were dispersed.

The aircraft is equipped with external flow-through devices which collect particulates on filter paper and on board among its crew are special equipment operators from the Air Force Technical Applications Center.

On operational sorties like today's from RAF Mildenhall the crew is normally minimized to pilots, navigator, and special equipment operators, to reduce radiation exposure to mission-essential personnel only.

In a statement on Monday, the CTBTO said: 'If a nuclear test were to take place that releases I-131 it would also be expected to release many other radioactive isotopes.

'Thus the CTBTO measures isotopes. No other nuclear fission isotopes have been measured at elevated levels in conjunction with I-131 in Europe so far.'

The organisation, which operates a worldwide monitoring system, said that it was not concerned about the reports of Iodine-131 in Europe.

'No detections above typical local historical levels have been observed,' the CTBTO said.

The deployment of the WC-135 aircraft, which detects and identifies explosions from the air and was used after the Chernobyl disaster in the Soviet Ukraine in 1986, adds weight to the argument.

The plane was deployed to RAF Mildenhall in Britain (pictured) but it is still not clear exactly why it has been sent to Europe

The US Air Force plane was tracked flying into RAF Mildenhall in Suffolk last week on an undisclosed mission

WHAT COULD HAVE CAUSED THE SPIKE? A NUCLEAR REACTOR LEAK IN EASTERN EUROPE An air filter station in Svanhovd, Norway, was the first place to measure the Iodine-131 in the second week of January. Next it was measured in Rovaniemi, in Finnish Lapland. Within two weeks, it was traced in Poland, Czech Republic, Germany, France and Spain. This movement led the Norwegian Radiation Protection Authority (NRPA) to suggest the particles may have originated in Eastern Europe. It's possible that the particles could have come from an incident at a nuclear reactor. IODINE PLANT LEAK The compounds may also have also come from an Iodine plant. The isotope Iodine-131 is used in medicine to treat to thyroid problems and is produced commercially across Europe. RECENT EVENT The isotope has a half-life of only eight days, so whatever caused the spike must have happened in the first two weeks of January. 'It was rough weather in the period when the measurements were made, so we can't trace the release back to a particular location,' Astrid Liland, head of emergency preparedness at the NRPA, told the Barents Observer. Advertisement

It's arrival comes amid tense times between Russia and the West, with America’s highest ranking military officer General Joe Dunford comparing the political climate to that during the Cold War.

He said that his meeting with General Valeriy Gerasimov, his counterpart in the Kremlin, is 'absolutely critical' as the tension between the two nations verges on breaking point.

It comes after two Russian jets flew low over a Royal Navy destroyer docked off the coast of Romania in a show of force branded 'unsafe and unprofessional' by Navy officers.

And a Russian spy ship armed with surface-to-air missiles with a crew of 200 sailed within 30 miles of a key US submarine base on the Connecticut coastline.

Scores of people filmed a mysterious light travelling through the sky at the weekend and the US Navy released a statement saying its testing of two Trident missiles was 'not in response to any world events'

The US Navy have been contacted for comment on the WC-135 but it has not yet released any official comment on the purpose of its mission.

And while it is not unheard of for the planes to fly to Europe, missions are rare and its arrival coincides with the detection of Iodine-131.

It was first recorded in Norway and have now been found in Poland, Czech Republic, Germany, France and Spain.

The isotope has a half-life of only eight days, which suggests the particles must have entered the atmosphere after a recent event.

The pattern of movement of the particles suggests they may have originated in Eastern Europe, according to the Norwegian Radiation Protection Authority (NRPA).

The pattern of movement of the radioactive particles suggests they may have originated in Eastern Europe, according to the Norwegian Radiation Protection Authority (NRPA)

HOW THE 'SNIFFER' TESTS THE AIR The WC-135 is known as the 'sniffer' or 'weather bird' by its crews because of its unique role in the sky. It gathers effluent gasses with two scoops on the sides of the fuselage, which then trap fallout particles on filters that the crew can analyse in real time. They can then use the data to confirm the presence of nuclear fallout and possibly determine the characteristics of the warhead involved. It can use the materials in the air to confirm the type of explosion, for example, whether it is from a warhead or a power plant. The WC-135 can also be used to track radioactive activity, which it did after the Chernobyl disaster in the Soviet Union in 1986 and Fukushima in 2011. One was also deployed near North Korea in anticipation of Kim Jong-un's rocket launches. A WC-135 was also seen transiting into UK airspace in August 2013 raising speculation it was used in Syria after claims chemical weapons have been used. The plane has a maximum crew of 33. However, it usually flies with a minimal crew to lessen the risk of chemical exposure. Advertisement

'It was rough weather in the period when the measurements were made, so we can't trace the release back to a particular location,' Astrid Liland, head of emergency preparedness at the NRPA, told the Barents Observer.

'Measurements from several places in Europe might indicate it comes from Eastern Europe.

'Increased levels of radioactive iodine in air were made in northern-Norway, northern-Finland and Poland in week two, and in other European countries the following two weeks.'

She said it is difficult to pinpoint where the radioactive material came from.

It's possible that the particles could have come from an incident at a nuclear reactor.

An explosion at a plant run by French firm EDF – just 75 miles across the Channel – added to concerns over nuclear safety earlier this month.

Scientists are yet to explain where the radioactive material came from but the particles may have come from an incident at a nuclear reactor. Pictured above is the Chernobyl nuclear power plant

The company, which is planning Britain’s first nuclear power station in a generation, was forced to shut down its nuclear reactor at the Flamanville plant in Normandy after the blast caused a fire that left five people suffering from smoke inhalation.

WHY IS IODINE-131 DANGEROUS? Iodine-131 has a very short half life of just eight days, making it very radioactive. When it is present in high levels in the environment, it contaminates food. After it is swallowed it will accumulate in the thyroid. As it decays, it damages body tissue and can cause thyroid cancer. However levels present in the atmosphere today are too low to be damaging. Advertisement

But the compounds may also have also come from an Iodine plant. The isotope Iodine-131 is used in medicine to treat to thyroid problems and is produced commercially across Europe.

Iodine-131 can cause harm because it has a very short half life of just eight days, making it very radioactive.

When it is present in high levels in the environment, it can contaminate food and after it is swallowed it accumulates in the thyroid.

As it decays, it damages body tissue and can cause thyroid cancer.

However levels present in the atmosphere today are too low to be damaging, according to Ms Liland.

She said: 'We do measure small amounts of radioactivity in air from time to time because we have very sensitive measuring equipment.

'The measurements at Svanhovd in January were very, very low. So were the measurements made in neighbouring countries, like Finland.

'The levels raise no concern for humans or the environment.'