U.S. Republican presidential front-runner Donald Trump has called North Korean leader Kim Jong-un a "madman" armed with nuclear weapons, accusing the administration of President Barack Obama of ignoring the problem.

Trump made the accusation during a Republican presidential TV debate Tuesday night, talking about threats to the United States and blasting the landmark deal on Iran's nuclear program as "one of the worst contracts ever signed."

"First of all, it's not only Russia. We have problems with North Korea, where they actually have nuclear weapons. You know, nobody talks about it," Trump said. "We have somebody over there, a madman, who already has nuclear weapons. We don't talk about that. That's a problem."

Trump said China is also a problem both economically and in terms of "what they're doing in the South China Sea."

"They are becoming a very, very major force. So, we have more than just Russia," he said of China.

In August, Trump said the North's leader is either "mad" or "a genius," and much more "unstable" and difficult to deal with than his late father.

North Korea has conducted three underground nuclear tests so far, in 2006, 2009 and 2013. The country has also conducted a series of long-range missile and rocket launches since 1998. In its most recent launch in late 2012, the North succeeded in putting a satellite into orbit.

Analysts have warned that it is only a matter of time until the North develops nuclear-tipped missiles. Some experts have recently warned that the communist nation's nuclear arsenal could expand to as many as 100 bombs by 2020.

The six-party talks aimed at resolving the North Korean standoff have been stalled since late 2008. North Korea demands the unconditional resumption of negotiations, while the U.S. says that Pyongyang must first take concrete steps demonstrating its denuclearization commitments.

Trump has drawn keen attention from South Korean media for his repeated, unfounded criticism of the Asian ally for relying on the U.S. for its defense without giving the U.S. anything in return while making a lot of money.

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. (Yonhap)