A top aide to US National Security Advisor Michael Flynn has been forced out of a job in an ongoing row over Russian sanctions.

According to nine unnamed US officials, Flynn called Russia's ambassador to the US to discuss sanctions a month before President Donald Trump took office.

Though the content of the telephone conversations is not known, the Washington Post has alleged that Flynn discussed undermining the sanctions, which former President Barack Obama imposed, as a result of suspected Russian hacking.

The calls allegedly took place on 29 December 2016 – the day Obama imposed the sanctions.

Two sources claim Russian ambassador Sergey Kislyak was told by Flynn to tell Moscow not to overreact about the sanctions as they would be undone by Trump, claims the Washington Post's report.

"Kislyak was left with the impression that the sanctions would be revisited at a later time," a former official said to Politico.

Democrats have since called for Flynn to be sacked, questioning the legality of his alleged actions and – as a sign of growing tensions – his aide Robin Townley has been refused the security clearance required to carry out his role.

As a result, Townley, a former Marine intelligence officer had long maintained the highest security level clearance, will no longer be able to serve on the National Security Council (NSC).

No reason was immediately given for Townley's rejection. However, sources have told Politico it was done by the Central Intelligence Agency (CIA) to undermine Flynn.

"They believe this is a hit job from inside the CIA on Flynn and the people close to him," a source said to the outlet. It is not yet clear if Flynn will follow Townley out.

Flynn originally issued two strong denials to the allegations, but later issued a statement which said while "he had no recollection of discussing sanctions, he couldn't be certain that the topic never came up".

Spokespeople for the NSC and the CIA have declined to comment. The White House press office did not respond to Politico's request for comment.

The NSA's primary role is to brief and advise the sitting president on national security issues. They do not have direct command authority over law enforcement or the US military.

Michael Flynn
Michael Flynn was President Donald Trump's pick for US National Security Adviser. Alex Wong/ Getty Images