U.S. Republican presidential hopeful Donald Trump has said he thinks North Korean leader Kim Jong-un is either "mad" or "a genius," and much more difficult to deal with than his late father.

"We have this mad guy, I guess he's mad, either he's mad or he's a genius, one or the other, but he's actually more unstable, even than his father, they say. They said the father was a pleasure by comparison to him, in North Korea," Trump said in an interview Friday with the Birmingham-based radio station WAPI, according to the Internet news outlet Breitbart.

Trump said the situation on the Korean Peninsula is "heating up again," referring to heightened tensions between South and North Korea following an exchange of artillery fire last week amid fears the situation could lead to larger clashes.

The billionaire real estate mogul reiterated his criticism of South Korea that the Asian nation relies on the U.S. for its defense without giving the U.S. anything in return while making a lot of money, a charge that shows his ignorance about American security interests and policy.

"You know it's heating up again. So, we send our ships, and I think South Korea's great. I think it's wonderful. I just order 4,000 television sets for a job that I'm doing, right?" Trump said.

"And guess what? Between Samsung, and LG, and Sharp, they all come from South Korea, They're making a fortune. So, we send our troops, we're getting ready to go in there and defend them. We get nothing. It's like crazy," he said.

Sharp is not a South Korean company, but a Japanese firm.

Last month, Trump made a similar remark that whenever South Korea "is in trouble, our military takes care. You know we get nothing." In 2011, ahead of the 2012 presidential election, he also said that the U.S. is protecting South Korea, but "they don't pay us."

About 28,500 U.S. troops are stationed in South Korea to help defend the Asian ally from the communist North, a legacy of the 1950-53 Korean War that ended in a truce, not a peace treaty, which left the divided peninsula still technically at war.

Seoul has long shared the cost of stationing U.S. forces.

Last year, the two countries renewed their cost-sharing agreement, known as the Special Measures Agreement, with Seoul agreeing to pay 920 billion won ($886 million) for 2014 for the upkeep of the U.S. troops, a 5.8 percent increase from a year earlier.

Moreover, the American military presence on the peninsula is seen as in line with U.S. national interests in a region marked by a rising China.

Trump has emerged as a leading candidate for the Republican presidential nomination for next year's election as he made a series of unbridled remarks on immigration and other sensitive issues that apparently struck a chord with conservatives. (Yonhap)