Charges against Michael Flynn were unsealed on Friday morning, local time, and he entered a guilty plea at a hearing in a Washington court hours later.

He is the fourth person connected to the Trump campaign to be charged as part of Special Prosecutor Robert Mueller's probe into Russian interference in the 2016 US presidential election.

Trump's son-in-law and advisor Jared Kushner directed Flynn to contact Russian officials around December 22 about a UN resolution regarding Israel, NBC News reported on Friday.

Flynn, a retired United States army lieutenant general, served just 24 days as Trump's national security adviser between January and February 2017.

He was forced to resign when multiple reports claimed he had misled Vice President Mike Pence about his conversations with the Russian ambassador to the US, Sergey Kislyak, regarding US sanctions against Russia.

Flynn has been charged with wilfully making false statements to the FBI on two occasions in January 2017, four days after Trump's inauguration.

Flynn falsely stated that he did not ask Kislyak to refrain from escalating the situation after the US imposed sanctions on Russia, a court filing said.

He also falsely stated that he did not recall the Russian ambassador telling him that Russia had chosen to moderate its response to US sanctions as a result of his request.

The secret meeting with Kislyak took place in December, 2016 in Trump Tower with Kushner, Trump's son-in-law and senior adviser.

Agents for the FBI later asked Flynn in January whether he had talked with Kislyak about sanctions imposed by President Barack Obama in retaliation for Russia's election meddling.

Flynn, who as a private citizen during those conversations was barred from negotiating with foreign powers, told the agents that sanctions hadn't come up, the Washington Post previously reported.

The Justice Department then informed the White House that Flynn's denial contradicted the contents of phone calls intercepted by intelligence agencies, potentially exposing him to blackmail by Russia.

Following the guilty plea, the White House said Flynn was acting alone.

"Nothing about the guilty plea or the charge implicates anyone other than Mr Flynn," said Ty Cobb, a White House attorney, adding that the plea "clears the way for a prompt and reasonable conclusion" to the Russia probe.

"The false statements involved mirror the false statements to White House officials which resulted in his resignation in February of this year."

Flynn, 58, was forced to resign less than a month into Trump's term.

After he left the administration, he filed an updated foreign registration form showing that he hadn't disclosed multiple contacts and payments from foreign entities while serving as an campaign adviser to Trump starting in February 2016.

At the time, Flynn ran a consulting business and, in one case, the company received USD530,000 ($694,000) from Inovo BV, a Dutch company working on behalf of Turkey's government, to lobby the US for extradition of a dissident cleric who has opposed President Recep Erdogan of Turkey.

He also disclosed payments from online news site RT, described in an unclassified US intelligence report as "the Kremlin's principal international propaganda outlet," and Kaspersky Government Security, a cybersecurity business that US authorities say works closely with Russia's main intelligence agency, the FSB.

Flynn served 33 years with the military and rose to the top of its intelligence apparatus but he was fired by Obama in 2014 for bucking his military superiors. He then started a private lobbying and consulting practice that did business in foreign countries including Russia and Turkey.

Flynn, in a statement, said he accepts full responsibility for his actions.

"It has been extraordinarily painful to endure these many months of false accusations of 'treason' and other outrageous acts. Such false accusations are contrary to everything I have ever done and stood for," he said.

The charges suggest that Mueller's probe into Russian interference is intensifying.

A former low-ranking foreign policy adviser to the Trump campaign, George Papadopoulos, pleaded guilty in October to lying to the FBI regarding his attempts to get Russian officials to meet with Trump during the campaign.

Paul Manafort, a former campaign manager for Trump, and an associate, Rick Gates, have been charged with offences including conspiracy against the United States and money laundering relating to their lobbying work for Ukrainian political parties and leaders between 2006 and at least 2016.

Their charges do not mention the Trump campaign but the case arose out of Mueller's investigation.

Authorities have concluded that there was significant influence by Russia during the campaign, including hundreds of thousands of fake social media accounts that attempted to create division and conflict among voters by spreading false information.

Reuters, Bloomberg