Republican presidential candidate Marco Rubio took a few sharp shots at GOP presidential frontrunner Donald Trump on Thursday, calling the billionaire businessman a "touchy and insincere guy" who is not well-informed about key issues in the race.

"First of all, he takes shots at everybody that gets anywhere close to him, in terms of a poll, or anytime he hits a rough spot, that's what he does," said Rubio during an interview on Kentucky Sports Radio.

The Florida senator also challenged Trump's command of the issues.

"He had a really bad debate performance last week," said Rubio. "He's not well informed on the issues. He really never talks about issues and can't have more than a 10-second soundbite on any key issue. And I think he's kind of been exposed a little bit over the last seven days, and he's a very touchy and insecure guy and so that's how he reacts, and people can see through it."

Later that night, Rubio doubled down on his attack against Trump, telling Fox News host Bill O'Reilly that his opponent "can't have a conversation about policy because, quite frankly, he doesn't know anything about policy," reports ABC News. He added that Trump's "foreign policy apparently is a secret he can't tell us because he doesn't want to reveal it to the enemy."

Rubio's jabs marked the first time that the Republican senator has engaged in heavily criticizing Trump in the 2016 race. The Florida official may have been provoked when the reality television star slammed him while campaigning in South Carolina a day earlier. On Wednesday, Trump called Rubio a "lightweight" and poked fun at the Florida senator's perspiration during the second GOP debate last week.

In response to Rubio's criticism, the real estate mogul fired back, calling the 44-year-old senator a "kid" during an interview Thursday with CNN.

"Marco Rubio, he's like a kid. He shouldn't be running in this race as far as I'm concerned," said the 69-year-old tycoon.

Trump also downplayed Rubio's public service, saying, "Marco Rubio sits behind a desk; sometimes and he reads stuff, he's in committees so you know that's all he does. I create jobs all day long."

"Believe me, I'll know more about this than all of them put together," Trump added. "If Marco Rubio's good, how come we're doing so badly? He's a sitting U.S. senator so why doesn't he do something about it? I'm not in government, he is in government."

A new Quinnipiac national poll shows that Trump is still in the lead in the Republican field while Rubio is in fifth place.