It is not actually a phone, and it is not really red.

But in popular culture, the hotline between Russia and the US, formally known as the Washington-Moscow Direct Communications Link, is always referred to at the “red phone”.

This week, it was revealed that tensions rose between the US and Russia amid mounting evidence that Moscow had sought to influence the outcome of the presidential election, President Barack Obama used the so-called Red Phone to contact his counterpart, Vladimir Putin, and express his displeasure.

The 'red phone' is not actually a red phone (Wikipedia)

NBC News said that Mr Obama initially spoke to Mr Putin at the September G-20 G-20 summit in China, where he raised with him the hacking of Democratic National Committee emails - allegedly by hackers linked to the Russian government.

Not wanting to inflame the situation, Mr Obama reportedly used moderate language to warn Mr Putin of the consequences if Russian interference did not stop.

But when the hacking continued, Mr Obama resorted to the Cold War communication system to express his displeasure.

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“International law, including the law for armed conflict, applies to actions in cyberspace,” said part of a message sent over the red phone on October 31. “We will hold Russia to those standards.”

James Collins, a former US ambassador to Russia and now a fellow at the Carneie Endowment for International Peace, told The Independent the fact that Mr Obama spoke directly to Mr Putin - apparently saying “cut it out” - underscored the seriousness of the issue.

He said he believed the relationship between the two countries was probably worse than at any time since the end of the Cold War and that the two were involved in a series of “tit-for-tat” actions.

“I think the idea that Russia played whatever role it did, in the electoral process was something that struck a very strong chord among Americans,” he said.

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The network said that the red phone system, used to communicate in moments of crisis such as the September 11 attacks, was never actually a phone, but rather progressed from teletype more than 50 years ago, to the fax and now to email. The communication links the two counrties Nuclear Risk Reduction Centres.

NBC said that in 2013, the Obama administration added a channel intended to send email messages and attachments about cyber incidents. This is one of a series of cyber-related confidence-building measures designed to address the need for secure and reliable lines of communication about cybersecurity, it said.

The Obama administration had never used the cyber line before, officials said.

“It’s a dramatic step to pick that phone up and use it,” said retired Adm James Stavridis, the former head of NATO.

The CIA has reportedly conclude that Russia interfered in the election, with the intention of benefiting Donald Trump, who has spoken favourably of Mr Putin and of his wish of a reset in relations.

During the election campaign, he frequently used information contained in the DNC emails, which were published by Wikileaks, to attack Hillary Clinton.

Since the CIA’s conclusions were first published by the Washington Post, Mr Trump has rejected its finding. His campaign issued a statement saying the agency was made up of the “same people” who claimed Saddam Hussein possessed weapons of mass destruction.