US President Donald Trump's first trip to a combat zone may have been shrouded in secrecy, but aviation enthusiasts were able to track Air Force One on its flight to Iraq — to the concern of US military veterans.

Mr Trump and first lady Melania Trump landed in an airbase west of Baghdad on Boxing Day night (local time) in an unannounced visit to greet US soldiers, after an overnight flight from Washington.

But hours earlier, plane enthusiast Alan Meloy spotted the VC-25 USAF plane flying over England, posting pictures of the blue and white aircraft to his Flickr account, and noting the call sign RCH358 being used for the craft.

The photo and the call sign were enough for plane trackers to speculate that the plane could be Air Force One, on a secret flight out of the US.

Only two VC-25 USAF aircraft are in service, and are both operated by the US Air Force as Air Force One.

The Twitter account tracking civilian and military aircraft of interest, @CivMilAir, followed the plane's journey from Joint Base Andrews in Maryland, over the UK and on to Hungary.

It suggested that when the craft was dormant for three hours that it might not have been refuelling but could have swapped codes or switched off its transponder — a claim also made by the official Wikileaks account which joined in on the speculation:

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The CivMilAir account noted reports from the White House press pool that no Marine was posted outside the West Wing in Washington, DC, suggesting Mr Trump may have been on the move, before news of the President's arrival in Iraq broke:

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The online tracking of Air Force One concerned Iraq and Afghanistan Veterans of America CEO Paul Reickhoff, who questioned the security protocols put in place by the White House:

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Reuters' political reporter Steve Holland, who flew on Air Force One's flight to Iraq, said the plane's shades were drawn and lights were out for much of the 11-hour trip to increase security.

Mr Trump said he was not personally concerned for his safety on the trip but he said he had never experienced a flight with such security measures in place.

"I had concerns for the institution of the presidency, not for myself personally," he said.

"I had concerns for the first lady — I will tell you.

"But if you would have seen what we had to go through … with the darkened plane, with all windows closed, with no lights on whatsoever; pitch black … I've never seen anything like it."