Republican presidential front-runner Donald Trump has expressed concern about the possibility of countries like North Korea using nuclear weapons.

"Nuclear is a major problem. And we have major problems, because you have other people that would be very fast on that. You look at North Korea, you look at some of these countries, I don't think they would hesitate to use it if they really had it in a proper manner," Trump said on CBS' "Face the Nation" on Sunday.

Trump has repeatedly called North Korean leader Kim Jong-un a "madman" with nuclear bombs, accusing the administration of President Barack Obama of ignoring the North Korean nuclear issue and not even talking about the issue.

Trump said that using nuclear weapons should remain "an absolute last stance."

"We have to be somewhat unpredictable in this whole thing.

Nuclear, though, has to be absolute last stance. Don't forget, I was against the war in Iraq. I'm not a fast trigger," he said Sunday. "You have guys that you would think are very low-key. They would be faster than me. I would be a very slow trigger with nuclear."

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 or 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. (Yonhap)