President Trump's son-in-law and senior adviser Jared Kushner talked with the Russian ambassador in December about establishing a backchannel for communications, ABC News has learned from two sources.
The Post says the president's son-in-law and point man on the Middle East spoke to Russia's ambassador to the USA about setting up the secure line in early December during a meeting at Trump Tower.
"I mean, we have back-channel communications with a number of - with a number of countries", McMaster said.
The Post report, citing anonymous U.S. officials who were briefed on intelligence reports on intercepted Russian communications, said Kislyak told his superiors that Kushner proposed using Russian diplomatic facilities for their discussions, apparently to make them more hard to monitor.
The sources stress that the talk between Kushner and the Russian envoy about communications was focused on the US response to the crisis in Syria and other policy-related matters. Kushner is under investigation as part of the FBI probe into Russian meddling in the 2016 election.
The Post reported earlier that investigators are focusing on meetings Kushner held in December with Moscow's ambassador and the head of a Russian bank that has been under USA sanctions since 2014.
"I am so proud of the future you are going to have", he said.
Donald Trump's top advisers said he accomplished what he set out to do on his first overseas trip as president: pressing North Atlantic Treaty Organisation to do more to fight terrorism, bringing home billions in business deals, and calling on European allies to drop trade restrictions.
Chief economic adviser Gary Cohn, a close Kushner ally, declined to answer questions about Kushner.
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Kislyak has been at the center of contacts between Trump administration officials and Russian Federation.
In that case, McMaster's "going before the press didn't do anything to limit" fallout from revelations that Trump disclosed code-word information to the Russians, said Jon Michaels, a professor and expert on national security at UCLA Law.
On Saturday, the AP confirmed that the Senate Intelligence Committee, which is investigating Russia's meddling in the 2016 USA presidential election, has requested information and documents Trump's campaign.
President Donald Trump says he'll make a final decision next week on whether the USA will stay in the Paris climate agreement.
It covers materials such as emails, phone records and documents dating to Trump's first days as a candidate in July 2015.
Those inquiries now include scrutiny of Kushner, according to the newspaper.
Reuters reported Friday that Kushner had at least three previously undisclosed contacts with Kislyak past year, including two phone calls between April and November. Kushner's attorney, Jamie Gorelick, told Reuters that Kushner "has no recollection of the calls as described".
"Hard to fully convey the gravity of this", said Susan Hennessey, a national security fellow at the Brookings Institution and a former lawyer for the National Security Agency, of the Washington Post report.
"Really, being spoken to, does not confer a target status on the individual", he said.