US Secretary of State John Kerry warned on Thursday that North Korea's expected nuclear tests only increase the risk of conflict and would do nothing to help the country's stricken people.

Kerry made the remarks when he dropped in on a group of young students taking a foreign policy classroom in the State Department.

"You look at the problems we're having with North Korea right now, questions of the imminency perhaps of another test, more missiles being fired, perhaps a nuclear test. To what end?" he asked.

"I mean, all that will happen is greater potential of conflict."

North Korea has vowed to carry out a third nuclear test soon, and concerns have been raised over the type of fissile material used in the device.

Tests by Pyongyang in 2006 and 2009 involved plutonium, so a uranium detonation would prove that regime of new leader Kim Jong-Un has developed an additional way to make bombs.

"The people of North Korea are starving," Kerry added, addressing the issue of North Korea for the first time since he took over as America's top diplomat late on Friday.

"They desperately need to become more open and connected to the world instead of harboring some of the worst gulags in the world where people are tortured, and forced labor," Kerry added.