Former Poland Prime Minister Donald Tusk has tweeted a photo of himself pointing a pair of fingers in an apparent "gun" gesture at US President Donald Trump's back.

"Despite seasonal turbulences our transatlantic friendship must last #Trump #NATO," Mr Tusk captioned the image posted on Thursday.

The newly elected president of the European People's Party — a conservative political party committed to strengthening the EU – ended his term as president of the European Council at the start of this month, after serving as Prime Minister of Poland from 2007-2014.

The photo of the pair – who had an tense relationship during the Brussels politician's term at the top of the EU – was taken at last year's G7 meeting in Canada, The Sun reports.

But it has now emerged as Mr Tusk takes a fresh swipe at the US leader.

The move by Mr Tusk — a frequent critic of Mr Trump — concerned many social media users, who alleged the post had a menacing undertone, with some apologising on his behalf.

"[Outgoing] European Union Council Leader Donald Tusk tweets himself holding 2 fingers against Trump's back like a GUN," tweeted user @justathoughtUSA. "Is that a THREAT?"

"We all know that the Globalist EU opposes Trump, but this goes too far," continued the tweet, which tagged the president and the FBI.

"As a Polish citizen I am embarrassed for this person," wrote user @RZbijowski.

"What a disgusting gesture of @donaldtusk," @Makaisia posted.

"This moron has no manners!!!"

Tusk waded into the diplomatic row which blew up at this week's NATO summit after Justin Trudeau and Emmanuel Macron were caught mocking Trump behind his back.

Tusk and Trump were regularly embroiled in a war of words over their differing views on Brexit and the EU and have had a fraught relationship.

In September, Tusk used a UN speech to slam the President's views as "false and dangerous" after just a month earlier calling Trump's desire for Russia to rejoin the G7 a "business calculation".

To protect the truth, it is not enough to accuse others of promoting fake news," he said, as reported by Politico. "Frankly speaking, it would be enough to stop lying."

Tusk also last year warned Trump to remember who is friends were, prompting the US President to blast: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"