Republican presidential candidate Donald Trump on Monday stepped up his attacks on rival Ted Cruz that focus on his Canadian birthplace, calling it a "problem" that the Texas senator must "settle" or drop out of the race.

"It's time for Ted Cruz to either settle his problem with the FACT that he was born in Canada and was a citizen of Canada, or get out of the race," Trump tweeted Monday morning.

It's the first time that the real estate mogul has called on Cruz to act or drop out of the race for the White House. For his part, Cruz has continually dismissed Trump's attacks, saying he is eligible for the presidency regardless of where he was born because his mother is a U.S. citizen.

"As you noticed, a number of very top constitutional lawyers have come and said - he was born in Canada. He didn't tell people. He said he didn't know about it. Until 15 months ago, he was a Canadian citizen. He was joint with the United States, but he was a Canadian citizen until 15 months ago," Trump said Sunday on CBS' "Face the Nation," according CBS News.

For much of the campaign season, Cruz and Trump were amicable toward one another. However, that began to change last month when audio was released from a private fundraising event in which Cruz questioned Trump's "judgement," The New York Times reported. That, coupled with Cruz closing the gap in certain polls, resulted in harder hits from Trump, including questioning Cruz's citizenship.

Trump also made personal attacks on Cruz's "likability" last week. "I don't think Ted Cruz has a great chance, to be honest with you," Trump said last week, ABC News reported. "Look, the truth is, he's a nasty guy. He was so nice to me. I mean, I knew it. I was watching. I kept saying, 'Come on Ted. Let's go, okay.' But he's a nasty guy. Nobody likes him. Nobody in Congress likes him. Nobody likes him anywhere once they get to know him."

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