Donald Trump holds surprise meeting with RNC in D.C.

Donald Trump held a private meeting with Republican National Committee officials at the party's headquarters in Washington, D.C. Thursday afternoon.

The GOP frontrunner tweeted that he had a "very nice" meeting with RNC head Reince Priebus during his nearly hour-long visit.

Trump was scheduled to be in the nation's capital to meet with his foreign policy team, but his appearance at the RNC came as a surprise.

An RNC source told NBC News the meeting was planned last week and was originally intended to be a short, informal stop merely to take advantage of Trump's previously planned trip to D.C. But the New York real estate mogul ultimately spent nearly an hour huddling with GOP officials Thursday for a discussion that the source said included party unity and "a few other things" Priebus had hoped to discuss.

See Donald Trump on the campaign trail:

"The Chairman and Mr. Trump had a productive conversation about the state of the race. The Chairman is in constant communication with all of the candidates and their campaigns about the primaries, general election, and the convention. Meeting and phone conversations with candidates and their campaigns are common and will increase as we get closer to November," the RNC said in a statement.

A Trump source also told CNBC's John Harwood that the meeting primarily focused on the the GOP convention, financing and general election activities rather than on Trump's clashes with the party.

Trump has had an often contentious relationship with the RNC, arguing that the Republican Party has failed to treat him "fairly."

This week, Trump said he feels he is no longer bound to the pledge he made to the party to support the GOP nominee.

"No, I don't anymore. No. We'll see who it is," he said when asked if he still promises to support whoever gets the nomination.

WATCH: Trump leaves surprise meeting with RNC:

The meeting comes the day after Trump faced harsh criticism from both sides of the aisle for suggesting in an interview with MSNBC that he advocates "some form of punishment" for women who receive abortions.

After coming under fire from anti-abortion groups, Trump amended that position within hours, saying in a statement that "the doctor or any other person performing this illegal act upon a woman would be held legally responsible, not the woman."

Also on Thursday, Trump picked up the backing of another Washington insider — former New Hampshire Rep. Bill Zeliff, who previously supported New Jersey Gov. Chris Christie in the presidential race.

Zeliff confirmed to NBC News in an interview that he delivered a letter expressing his support to the Trump campaign about two weeks ago through Rep. Duncan Hunter, who was named Wednesday as one of Trump's House Leadership Committee chairmen.

Trump has the endorsement of only seven sitting members of Congress, one senator and three governors, as well as three former members. Zeliff's backing brings that number to four.