NEW YORK — His inauguration just days away, President-elect Donald Trump is continuing to lash out at critics in the intelligence community and Democrats in Congress who are vowing to skip his swearing-in ceremony.

The tough-talking Republican questioned whether the CIA director himself was “the leaker of fake news” in a Sunday night tweet.

The extraordinary criticism from the incoming president came hours after CIA chief John Brennan charged that Trump lacks a full understanding of the threat Moscow poses to the United States, delivering a public lecture to the president-elect that further highlighted the bitter state of Trump’s relations with American intelligence agencies.

“Now that he’s going to have an opportunity to do something for our national security as opposed to talking and tweeting, he’s going to have tremendous responsibility to make sure that U.S. and national security interests are protected,” Brennan said on “Fox News Sunday,” warning that the president-elect’s impulsivity could be dangerous.

Trump shot back in a Twitter post Sunday, saying: “Oh really, couldn’t do much worse — just look at Syria (red line), Crimea, Ukraine and the buildup of Russian nukes. Not good! Was this the leaker of Fake News?”

The president-elect remained behind closed doors in his Manhattan high-rise for the weekend as his team worked to answer questions about his plans at home and abroad once he’s sworn into office on Friday.

Among Trump’s immediate challenges: the United States’ complicated relationship with Russia, crafting an affordable health care alternative that doesn’t strip coverage from millions of Americans and dealing with an assertion by Rep. John Lewis, a civil rights icon, that his election was not legitimate.

Meanwhile, several Democrats in Congress vowed to skip Trump’s inauguration.

Trump’s lieutenants pushed back hard, particularly against Rep. John Lewis, D-Ga., a civil rights legend who said Russian interference in Trump’s election delegitimizes his presidency.