Republican lawmakers should follow their conscience on whether to support Donald Trump in November's presidential election, U.S. House Speaker Paul Ryan has said in an interview, reflecting the party's unease over its White House candidate.

"The last thing I would do is tell anybody to do something that's contrary to their conscience. Of course I wouldn't do that," the Republicans' most senior elected official said in excerpts released on Friday of an NBC interview set to air on Sunday.

Some Republican leaders and lawmakers in the House of Representatives are struggling to get behind the New York businessman, who last month became the party's presumptive nominee for the Nov. 8 election.

After an initial delay, Ryan has said he will back Trump but he has also acknowledged deep differences with him. He denounced as textbook racism Trump's criticism of a Mexican-American judge and has also criticized Trump's proposal - reiterated after the massacre of 49 people in a gay bar in Orlando on Sunday - to temporarily bar Muslims from the United States.

As Republicans seek to keep control of both chambers of Congress, Trump's comments on such issues have also worried some lawmakers concerned about their own election prospects, particularly in close races. All 247 House Republican seats are up for grabs in the election.

Trump, who has welcomed support from Ryan, this week fired back at Republican leaders, telling them to stop speaking out against him or else risk him potentially running "by myself."

Ryan said earlier this week at his weekly press conference that he does not plan to withdraw his support of Trump, although they disagree on some key issues.

"I feel as a responsibility institutionally as the speaker of the House that I should not be leading some chasm in the middle of our party. Because you know what I know that'll do? That'll definitely knock us out of the White House," Ryan said in the interview for NBC's "Meet the Press" program.

Trump's embrace this week of the lesbian, gay, bisexual and transgender community and potential gun control measures, in contrast to general Republican orthodoxy have also thrown conservative lawmakers.

Still, Republican leaders have to reconcile their unease with the fact that primary Republican voters opted for Trump. He has never held elected office before but won more than enough delegates to secure the party's nomination at the Republican convention in July.