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

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.

"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.

The current administration of South Korean President Park Geun-Hye is adamant that substantive inter-Korean talks can only begin once the North makes a tangible commitment to denuclearisation.

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.

North Korea held its first ruling party congress for nearly 40 years earlier this month, formally endorsing leader Kim Jong-Un's policy of expanding the country's nuclear arsenal.

The congress was seen as an elaborate coronation for the 33-year-old Kim, securing his status as supreme leader and confirming his legacy "byungjin" doctrine of twin economic and nuclear development.

The UN Security Council in March imposed its toughest sanctions to date following the January nuclear test and a long-range rocket launch the following month.

The North had already been subject to a range of sanctions because of earlier nuclear tests -- in 2006, 2009 and 2013 -- and a series of long-range missile tests presented as satellite launches.

The Security Council resolutions condemned the nuclear tests and rocket launches as a threat to international peace and security.

North Korea has challenged the sanctions, arguing that no international laws or agreements stipulate that nuclear tests are such a threat.

It has also noted that the Security Council never sanctioned the United States or other countries for their nuclear tests or ballistic missile launches.

Its long-held argument is its push for a nuclear deterrent has been forced by US hostility.