While wishing a Merry Christmas to troops overseas, President Donald Trump ventured into politics Tuesday by denying collusion with Russia during the 2016 election, defending his 2017 dismissal of FBI Director James Comey, and saying he doesn't know when the ongoing government shutdown will end.

"It’s a disgrace what’s happening in our country," Trump told reporters in the Oval Office. "But other than that, I wish everybody a very Merry Christmas."

While congressional Democrats say they will not support money for a border wall, Trump said the U.S. needs "a wall, a fence, whatever they want to call it." The remarks underscore how dug in both the White House and congressional Democrats have become amid a partial government shutdown that began Friday at midnight.

"The people of this country want border security," Trump told reporters on the fourth day of the shutdown caused by a standoff over the president's demand for funding of a wall along the U.S.-Mexico border.

Responding to reporters' questions after a series of Christmas calls to troops, Trump also:

Again attacked the Federal Reserve by saying, "They are raising interest rates too fast, that's my opinion." Trump has stepped up his criticism of the Central Bank in recent weeks amid a major selloff on Wall Street, including a 653-point plunge on the Dow Jones Industrial average on Christmas Eve.

Criticized the ongoing investigation into Russian efforts to influence the 2016 election, and denied his campaign had anything to do with it. "After two years, no collusion," Trump said. "There has been collusion but it's been by the Democrats."

Denied he tried to obstruct the Russia investigation through his dismissal of Comey, claiming Democrats also opposed the FBI director. "I fired him and everybody said, 'Why did you fire him, why did you fire him?'" Trump said. "Literally the day before I fired him, they were saying he should be fired."

Before the questions, Trump spoke by video conference "with service members stationed at remote sites worldwide to thank them for their service to our Nation," said a White House statement.

Trump placed calls to service members from all five branches. Recipients were stationed in Guam, Bahrain, Qatar and Alaska.

Trump spent Christmas in the White House, having postponed his end-of-the-year vacation in Florida because of the shutdown. The calls came at a time when Trump has launched a major reshuffling of U.S. military strategy, including pulling more than 2,000 U.S. troops out of Syria and changing top leadership at the Pentagon.

Earlier in the day, Trump tweeted a more general message: "Merry Christmas!" The president also tweeted a video message from him and first lady Melania Trump.

The traditional messages followed a Christmas Eve tweetstorm in which Trump posted more than a dozen times on various political topics.

The president criticized Democrats over the government shutdown and wall funding, pushed back against senators who criticized his withdrawal of U.S. troops from Syria, again attacked the Federal Reserve over higher interest rate hikes, and touted a briefing on North Korea policy.

"I am all alone (poor me) in the White House waiting for the Democrats to come back and make a deal on desperately needed Border Security," Trump tweeted at one point.

Later in the day, the first lady – who traveled back from the couple's Mar-a-Lago estate in Florida – and the president answered calls from children who were keeping tabs on Santa Claus via NORAD Tracks Santa. The couple then made an unscheduled visit to the National Cathedral for Christmas Eve services.

"Are you still a believer in Santa Claus?" Trump said to a 7-year-old named Coleman. '''Cuz at 7 it's marginal, right?"

The couple then made an unscheduled visit to Washington National Cathedral for Christmas Eve services.