At the time of the December meetings, Trump had already won the election. Contacts between people on the transition team and foreign governments can be routine, but the meetings and phone calls with the Russians were not made public at the time.

In early December, Kushner met Russian ambassador to the US, Sergey Kislyak, in New York, and he later sent a deputy to meet Kislyak.

Flynn was also present at the early December meeting and, later that month, Flynn held a call with Kislyak to discuss US-imposed sanctions against Russia. Flynn initially mischaracterised the conversation even to the Vice-President - which ultimately prompted his ouster from the White House.

In December, Kushner also met Sergey Gorkov, the head of Vnesheconombank, which has been the subject of US sanctions following Russia's annexation of Crimea and its support of separatists in eastern Ukraine.

In addition to the December meetings, a former senior intelligence official said FBI agents had been looking closely at earlier exchanges between Trump associates and the Russians dating back to the spring of 2016, including one at the Mayflower Hotel in Washington.

Kushner and Kislyak - as well as close Trump adviser and current Attorney-General Jeff Sessions - were present at an April 2016 event at the Mayflower where then-candidate Trump promised in a speech to seek better relations with Russia. It is unclear whether Kushner and Kislyak interacted there.

The New York Times reported that Kushner omitted his December meetings with Kislyak and Gorkov from security clearance forms, although his lawyer said that was mere error and he told the FBI soon after that he would amend the forms. The White House said that his meetings were normal and inconsequential.

Kushner has agreed to discuss his Russian contacts with the Senate Intelligence Committee, which is conducting one of several investigations into Russian meddling in the 2016 election.

In many ways, Kushner is a unique figure inside the White House. He is arguably the President's most trusted adviser, and he is also a close member of the President's family. His list of policy responsibilities is vast - his foreign policy portfolio alone includes Canada and Mexico, China and peace in the Middle East - yet he rarely speaks publicly about any of them.

Former FBI director Robert Mueller is now leading the probe into possible co-ordination between the Kremlin and the Trump campaign, and he has set up shop in the Patrick Henry Building in downtown District of Columbia. Even before he was picked by Deputy Attorney General Rod Rosenstein to take over the case, investigators had been stepping up their efforts - issuing subpoenas and looking to conduct interviews, people familiar with the matter said.

A small group of legislators, known as the Gang of Eight were recently notified of the change in tempo and focus in the investigation at a classified briefing.

It is unclear exactly how Mueller's leadership will affect the direction of the probe. This week, Justice Department ethics experts cleared him to take over the case even though lawyers at his former firm, WilmerHale, represent several people who could be caught up in the matter, including Kushner, Manafort and Trump's daughter Ivanka, who is married to Kushner.

Mueller resigned from the firm to take over the investigation.

Investigators are continuing to look aggressively into the dealings of Flynn, and a grand jury in Alexandria, Virginia, recently issued a subpoenas for records related to Flynn's businesses and finances, according to people familiar with the matter.

Flynn's company, the Flynn Intel Group, was paid more than $US500,000 by a company owned by a Turkish American businessman close to top Turkish officials for research on Fethullah Gulen, a cleric who Turkey's President believes was responsible for a coup attempt last summer. Flynn retroactively registered with the Justice Department in March as a paid foreign agent for Turkish interests.

Separately from the probe now run by Mueller, Flynn is being investigated by the Pentagon's top watchdog for his foreign payments. Flynn also received $US45,000 to appear in 2015 with Russian President Vladimir Putin at a dinner for RT, a Kremlin-controlled media organisation.

The Washington Post