House Speaker Paul Ryan (R-Wis.) on Thursday warned that his endorsement of Donald Trump shouldn't be seen as a blank check.

"If I hear things that I think are wrong, I'm not going to sit by and say nothing, because I think I have a duty as a Republican leader to defend Republican principles and our party's brand if I think they're being distorted," Ryan told Green Bay’s WTAQ radio.

Asked whether there are situations that could cause him to withdraw his support of Trump for president, Ryan responded, “of course there are.” 

"I'm not going to get into the speculation or hypotheticals. None of these things are ever blank checks. That goes with any situation in any kind of race. But right now, he won the thing fair and square," Ryan said. 

Trump's decision to tell The Washington Post that he wasn't ready to endorse Ryan in his reelection bid angered GOP leaders, including Republican National Committee Chairman Reince Priebus. 

It also exacerbated an already tough week for Trump, who over the weekend feuded with the parents of a Muslim U.S. soldier killed in the Iraq War. That controversy and others, along with polls showing Democratic presidential nominee Hillary Clinton enjoying a sizable post-convention bounce, have raised questions about whether Republican leaders will decouple themselves from Trump.

During the interview, Ryan repeated his criticism of the GOP nominee's comments regarding the family of Capt. Humayun Khan, calling them "beyond the pale." He didn't say he would pull his endorsement of Trump but vowed to speak out when necessary to distance his party from some of the candidate's more controversial comments.

"I don't like doing this. I don't want to do this, but I will do this because I feel like I have to in order to defend Republicans and our principles, so people don't make the mistake of thinking we think like that," Ryan said. 

And he said it's "distressing" that the controversies have deflected attention from attacking Clinton.

"He's had a pretty strange run since the convention. You think you'd ought to be focusing on Hillary Clinton and all her deficiencies," Ryan said.

"She is such a weak candidate that one would think we would be on offense against Hillary Clinton, and it is distressing that that's not what we are talking about these days."