Mitt Romney has asked his top advisers to find a way to block Donald Trump from being nominated at the Republican National Convention, according to a new report late Thursday.

A source told CNN that the former 2012 GOP presidential nominee is considering ways to keep the 2016 Republican front-runner from locking down the 1,237 delegates he would need to win the July convention.

"It sounds like the plan is to lock the convention," the source said.

The source added that Romney's advisers are trying to explore how a convention fight would take place, and what rules might have to be put in place to block Trump. Romney indicated Thursday that he is not looking to run himself.

But amending convention rules to prevent Trump from getting the nod may brew a bigger problem for the party if Trump were to respond by running as a third-party candidate, which would force conservatives to choose between the GOP-aligned candidates and likely lose the election to the Democrat. Trump warned Thursday that if he somehow were not to become the nominee, the GOP would lose thousands of votes around the country.

Romney's instruction comes a week into a battle he's waged with Trump. On Wednesday, Trump pushed back by posting a video on his social media channels that showed the former Massachusetts governor sharing left-leaning views on various policy issues.

On Thursday, Romney called Trump a "fraud," and Trump replied by saying Romney "chickened out" of running for the White House a third time.