North Korea on Monday issued a fresh call for the United States to halt military exercises with South Korea if it wants Pyongyang to suspend nuclear weapons tests.

"I wish the US government (will) study our proposal properly and provide answers to the proposal," Ri Yong-Ho, the North's chief nuclear negotiator, told reporters after two days of unofficial talks with former US diplomats and security experts in Singapore.

Stephen Bosworth, the former special envoy for North Korea policy who led the US side, stressed that he and his colleagues met with the North Koreans as private citizens and did not come with any proposals from their government.

"We came here to listen," he said after two days of talks with Ri's delegation at a luxury hotel in Singapore.

"We will have discussions when we get back but I would like to emphasise that we are not representing the US government or carrying any messages or proposals."

The United States earlier this month rejected the proposal, which had been passed to the US side through a "relevant channel", according to North Korea's official KCNA news agency.

US State Department spokeswoman Jen Psaki described the proposal as an "implicit threat" and accused the North of "inappropriately" linking routine military exercises between Washington and Seoul to the possibility of a nuclear test.

Ri told reporters in Singapore that the root cause of tensions on the peninsula are large-scale annual military exercises involving US and South Korean troops.

He said the North Koreans provided "detailed information of the intention and purpose" of their government's offer at the Singapore meeting.

"Part of the proposal is that if (the) United States puts a temporary halt to this large scale joint military exercise... we are ready to respond to the measures by putting a moratorium on the nuclear tests," Ri said through an interpreter.

- Military tensions -

The United States, which has close to 30,000 troops permanently stationed in South Korea, conducts a series of annual joint military exercises with its key Asian ally.

Seoul and Washington insist the drills are defensive in nature, but they are regularly condemned by Pyongyang as provocative rehearsals for invasion.

The annual exercises habitually raise military tensions between the two Koreas, who have remained technically at war since the 1950-53 Korean War concluded with a ceasefire rather than a peace treaty.

Bosworth said the US side emphasised the need to resume official discussions under the so-called Six Party Talks process aimed at ending North Korea's nuclear programme. They involve China, the United States, Japan and Russia as well as the two Koreas.

The process has stalled since 2008 after Pyongyang walked away from the talks.

Bosworth also said that the recent cyber attack on Hollywood movie studio Sony Pictures came up in the discussions but gave no further details.

US President Barack Obama has authorised a new layer of sanctions on several Pyongyang institutions and officials after US investigators said the North was behind the November attack.

North Korea has repeatedly denied involvement.

The talks in Singapore were held under the so-called "Track 2" diplomacy process which involves unofficial contacts. The countries do not have diplomatic ties.