U.S. President Barack Obama took aim at North Korea Thursday, May 26 calling it a "big worry" after a meeting with his G7 counterparts, as tensions escalate following Pyongyang's series of nuclear tests, AFP reports.

Obama made the remark at a briefing on the sidelines of Group of Seven talks in Japan where North Korea's provocations and its nuclear ambitions are among the topics on a packed agenda.

"North Korea is a big worry for all of us," Obama said, according to AFP.

"It is not the thing that poses necessarily the most immediate risk. (But) when you have such an unstable regime that is so isolated, that poses the kind of medium-term threat that we have to pay a lot of attention to."

Tensions between North and South Korea have been running high since Pyongyang conducted its fourth nuclear test in January.

In recent weeks, the North has made repeated proposals for military talks aimed at de-escalating the situation -- but the South has dismissed the offer as an "insincere" propaganda ploy.

Also Thursday, UN Secretary General Ban Ki-moon urged a return to talks with North Korea, during a visit to South Korea that has fuelled speculation of his presidential ambitions in his home country.

"We must find the path back to dialogue," Ban said at a peace and security forum.