President Barack Obama saluted a pair of United States Marines on Tuesday while holding what appeared to be a styrofoam cup in his saluting hand, a breach of military regulations that won't win him fans among veterans and servicemen.

Obama is known for drinking tea, not coffee, when he travels, especially before delivering speeches.

His quasi-saluting gesture was an instant embarrassment for the White House, and not because his environmentally taboo drinkware clashed with the green-policy speech he was on his way to deliver at a UN Climate Summit.

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Americans weren't amused after the White House posted a short video clip showing the commander-in-chief casually saluting a pair of US Marines while holding a drink

Americans, including those who have served in uniform, reacted harshly on Twitter after Obama's salute faux pas

JUDGMENT: With no news photographers catching the embarrassing moment, some are scratching their heads about why the White House published the video clip on its own

Saluting military personnel is something presidents are expected to do since they're at the top of the Pentagon's chain of command.

It's customary for a commander-in-chief to salute uniformed servicemen and women who salute him first as a sign of respect.

President Ronald Reagan reportedly began the tradition and future presidents, both Republicans and Democrats, followed suit.

But Obama's casual approach to the moment as he left his Marine One chopper at a helipad in New York City came at a time when he had just ordered U.S. fighter planes to fly perilous missions in the skies over Syria.

Twitter predictably erupted, with most commenters expressing outrage and a few insisting that it wasn't a major mistake.

Arriving: Marine One lands in New York City (above) as President Obama heads to deliver remarks at the United Nations Climate Summit

Keynote:President Obama (above) called on everyone in the international community to cut their carbon pollution during his speech

Real estate mogul and long-time Obama critic Donald Trump weighed in on the controversy, proclaiming the president's coffee cup salute 'terrible' in response to a tweet from one of his followers.

The 'Customs and Courtesies' manual of the U.S. Marine Corps advises that there are circumstances in which it's not appropriate to render a salute.

One of them is when military personnel are 'carrying articles with both hands or being otherwise so occupied as to make saluting impractical.'

A Yahoo News reporter mocked the president, speculating on the Starbucks size of his salute

His two cents: In response to a tweet from one of his followers about the coffee cup salute, Donald Trump proclaimed Obama's faux pas 'terrible!'

U.S. MILITARY MANUALS ON PROPER SALUTES U.S. Army Field Manual: 'Raise the right hand sharply, fingers and thumb extended and joined, palm facing down, and ... touch the tip of the right forefinger to the forehead near and slightly to the right of the right eyebrow.' U.S. Marine 'Customs and Courtesies' 'In some situations, the salute is not appropriate. In general, do not salute when ... carrying articles with both hands or being otherwise so occupied as to make saluting impractical.' Advertisement

The U.S. Army Field Manual describes the correct procedure for rendering a salute.

It instructs that when 'uncovered' – that is, not wearing a cap – one should 'touch the tip of the right forefinger to the forehead near and slightly to the right of the right eyebrow.'

During Tuesday's salute, Obama's cup was the only thing that made contact with his head.

Obama made saluting news once before, in 2013, when he forgot to salute a Marine guarding his chopper before he bounded up the stairs to board it.

Instead of sticking his head out the door and saluting, Obama returned all the way to ground level – and shook the young man's hand instead.

No wire photographers captured pictures of the exact moment when the president executed his unconventional tea cup maneuver on Tuesday, but online fury built after the White House itself posted a short video clip of it on its Instagram page.