Top US Democrats said Tuesday that White House National Security Advisor Mike Flynn may have violated the law in taking payments from Russia's RT television to attend a party with Vladimir Putin.

In a letter to the Pentagon, the senior Democrats of six powerful House committees asked the Pentagon for documents on Flynn's acceptance of payments from RT, which they called the "propaganda arm of the Russian government," after his retirement from the US Army in 2014.

The Democrats cited US laws and Pentagon policy against officials, including retired officers in the military's case, accepting direct or indirect payments from foreign governments.

They noted that after Flynn retired as a lieutenant general, he regularly appeared as a commentator on RT and has himself said he was paid to attend an RT gala dinner in December 2015 in which he sat with the Russian president.

They cited the Emoluments Clause of the US Constitution, which bans senior officials from taking money or gifts from foreign countries.

"The Department of Defense has made clear that this restriction applies to retired military officers because they continue to hold offices of trust," they wrote.

Flynn's final position in his three-decade military career was director of the Defense Intelligence Agency.

He was named as President Donald Trump's top security advisor despite strong criticism of his allegedly close relationship with Moscow and pictures from the RT gala showing him sitting next to Putin.

US intelligence agencies said in a report in January that RT was closely controlled by the Kremlin.

The Democrats asked the Pentagon to supply any communications between Flynn and Russian or other foreign officials, and information on payments he may have received from foreign sources.

Those signing the letter included the heads of the House Intelligence, Foreign Affairs and Armed Service Committees.