The national security advisor to the President of the United States resigned Monday following a controversy over his calls to the Russian ambassador to the US, and for failing to properly brief senior government officials about them.

 "Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect (Mike Pence) and others with incomplete information regarding my phone calls with the Russian Ambassador (Sergey Kislyak)," said Michael Flynn in his resignation letter Monday.

In the conversations that took place before Donald Trump's inauguration on January 20 and which were intercepted by the FBI, Flynn spoke of sanctions - imposed by former US President Barack Obama - against Russia for its alleged interference in November's presidential elections.

Flynn would have hidden from Pence and other senior officials these conversations, as the vice president had dismissed accusations of any such contacts before the media a month ago.

In his resignation letter, Flynn defended the conversations Monday saying such calls are a standard practice and are aimed "to facilitate a smooth transition and to build necessary relationships between the President, his advisers and foreign leaders."

Flynn, however, is no stranger to controversies; during the electoral campaign - before being named security adviser - he had used the social media to spread false news that were published by extreme right portals, about alleged crimes committed by Hillary Clinton, Trump's rival in the presidential race.  

He was indeed one of the first high-ranking figures to endorse Mr Trump, startling his brother officers by leading a chant against Hillary Clinton of “Lock Her Up” at the Republican National Convention in 2016, as reported in The Economist.

Mr Flynn already faced questions about ties with Russia after being fired by President Barack Obama as head of the Defense Intelligence Agency (DIA). Mr Flynn left that agency in 2014, claiming that he was fired for raising uncomfortable questions about the Obama government’s approach to fighting Islamic terrorism (Team Obama called him an insubordinate obsessive and bad manager). In 2015 the former DIA chief turned up in Moscow at a gala for the state propaganda outlet, Russia Today, sharing a table with President Vladimir Putin, reported The Economist.

After accepting Flynn's resignation, Trump appointed Retired Lieutenant General Joseph Keith Kellogg as the acting national security advisor on Monday.

Kellogg had served in the US Army from 1967 to 2003 and fought in the Vietnam War, and had received multiple medals.