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National Security Advisor Mike Flynn discussed hacking-related sanctions with the Russian ambassador before the Trump administration took office, contrary to the public assertions of Vice President Mike Pence and White House spokesman Sean Spicer, a U.S. intelligence official told NBC News.

The official said he was told there was no quid pro quo and that there has been no finding inside the government that Flynn did anything illegal.

But he said he was surprised when Flynn initially denied to the Washington Post, which first reported this story, that he discussed the sanctions on Russia with the ambassador. His spokesman later said he didn't recall whether he did, according to the Post.

On Friday, a spokesperson for Flynn told NBC News that Flynn "can't be 100 percent sure," but doesn't remember talking sanctions.

The FBI and intelligence officials became aware of the contents of the conversations because they were monitoring the communications of Russian diplomats, current and former officials have said.

Democrats are furious over the disclosure, a senior congressional staffer said, and critics, such as foreign policy scholar David Rothkopf, are accusing Flynn of having "lied." The White House did not immediately respond to NBC News's request for comment.

Flynn's contacts with Sergey Kislyak, Russia's ambassador to the U.S., were initially seen by critics as a potential violation of a 1799 law called the Logan Act, which prohibits a private citizen from negotiating with a foreign power in a dispute with the United States. But that law has rarely if ever been cited in a prosecution.

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The salient issue, critics say, is whether Flynn gave the Russian envoy any indication that the Trump administration would lift sanctions that were imposed to punish Russia for hacking and leaking emails during the U.S. presidential election, which intelligence officials say was meant to hurt Hillary Clinton and tip the election to Donald Trump.

The Russians did not respond as expected to the expulsion of 35 Russian diplomats from the U.S. as part of the sanctions. President Putin said he would not retaliate with expulsions of U.S. diplomats, and instead invited the children of diplomatic personnel in Russia to a Christmas and New Year's party.

As a candidate and as president-elect, President Trump repeatedly expressed doubt about whether Russia had interfered in the election, despite a unanimous "high confidence" assessment of the intelligence community. He later acknowledged Russian's role, but he has continued to praise Russian President Vladimir Putin.

Flynn spoke to Kislyak on Dec. 29, the same day the sanctions were announced, the White House has said, adding that the two also communicated by text message. White House spokesman Sean Spicer said the conversations were about routine matters such as scheduling a phone call between Trump and Putin. Spicer said the topics did not include the sanctions.

Vice President Mike Pence, appearing on the CBS program "Face the Nation," said Flynn and the Russian ambassador "did not discuss anything having to do with the United States's decision to expel diplomats or impose censure against Russia."

An administration official told NBC News that Vice President Pence's only source for saying Flynn did not discuss sanctions with the ambassador was Flynn himself.

The Post, which based its new reporting on nine sources, said Flynn twice denied on Wednesday that he discussed sanctions with Kislyak, but then changed his answer on Thursday.

A spokesman for the Russian Embassy in Washington, D.C., told NBC News that the embassy "does not comment on our conversations with local contacts — any local contacts — which are being conducted every day. We respect them."

Flynn, a retired Army general who ran the Defense Intelligence Agency until he was forced out early during the Obama administration, has been under fire for his paid relationship with the television network RT, which is funded by the Russian government.

He said he was paid to appear at a 2015 RT dinner in Moscow, but he hasn't said how much. He likened RT — which U.S. intelligence agencies say is a Russian propaganda outlet — to CNN and MSNBC.

Last week a group of House Democrats sent a letter to Defense Secretary James Mattis calling for a Pentagon investigation into the matter, given Flynn's status as a retired general.

On Friday, Rep. Adam Schiff, ranking member of the House Intelligence Committee said the possibility that Flynn discussed sanctions with Kislyak "raises serious questions of legality and fitness for office. If he did so, and then he and other administration officials misled the American people, his conduct would be all the more pernicious."

A few Democrats issued statements calling for Trump to fire Flynn.

"It's clear that concerns about General Flynn's ties to Russia were well warranted," said Rep. Eliot Engel of New York, ranking member of the House Foreign Affairs Committee. "The President must relieve General Flynn immediately."

Others called for further investigation.

"That General Flynn may have misled the vice president of the United States and others raises the most serious questions about his ability to continue serving as national security advisor," said Sen. Ed Markey of Massachusetts, a Senate Foreign Relations Committee member. "I hope federal law enforcement professionals investigate this matter immediately. Any effort to undermine our nation’s foreign policy – even during a transition period — may be illegal and must be taken seriously."