North Korea is making progress on an intercontinental ballistic missile capable of delivering a first-generation nuclear warhead to the continental United States, a leading U.S. think-tank said Tuesday.

The closely followed 38 North website of the Johns Hopkins University's U.S.-Korea Institute argued that ICBM mock-ups seen at recent military parades in Pyongyang were "less fake" than originally believed.

North Korean leader Kim Jong-Un's aunt fled to the United States while looking after her nephew during his time at a Swiss boarding school in the 1990s, a report said Tuesday.

Ko Yong-Suk, who effectively vanished in 1998, had in fact been granted U.S. political asylum, South Korea's JoongAng Daily said, citing a former senior intelligence official and a South Korean diplomat based in Berne at the time.

A group of South Korean lawmakers crossed the fortified border into North Korea Wednesday to visit a jointly run industrial park that recently re-opened after military tensions caused a five-month closure.

The rare cross-border trip by the 21 MPs comes as the two Koreas are struggling to build some momentum behind a series of stop-start confidence-building measures.

The head of a U.N. inquiry into North Korea rights abuses told Tuesday how he had been reduced to tears by witness accounts of people who fled the hardline state.

Michael Kirby, a veteran Australian high court judge, said the treatment of women returned to North Korea after already being abused in China had been particularly tough.

North Korea has undertaken major construction work at its main missile launch site, possibly to cater to larger and more mobile weapons, a U.S. think-tank said Tuesday.

Satellite images taken earlier this month suggest construction of a second flat mobile launch pad at the Sohae missile site, the U.S.-Korea Institute at Johns Hopkins University posted on its 38 North website.

South Korea on Monday deployed destroyers, jet fighters, attack helicopters and rocket launchers in a major live-fire drill against the threat of North Korea landing troops, the defense ministry said.

"The joint live-fire drill was aimed at repelling landings by enemy forces along the east coast," a ministry spokesman told Agence France Presse.

Seoul returned four North Korean fishermen and their boat Sunday after it drifted into southern waters, military authorities said, two days after Pyongyang handed back six detained South Koreans.

The vessel was picked up by a South Korean navy ship on Saturday after being spotted stranded off the country's east coast following engine failure, officials said.

North Korea returned six detained South Koreans across the two rivals' heavily-militarized land border on Friday, in a move that may help relax tensions.

The handover took place at 4:50pm (07:50 GMT) at the Panmunjom truce village where the armistice was signed ending hostilities in the 1950-53 Korean War, the South's Unification Ministry said.

North Korea will return six unidentified South Koreans across the two countries' heavily-militarized land border, it said in a surprise announcement Thursday.

The South's Unification Ministry said it had been informed the transfer would take place on Friday, but was given no details of the identities of the people being returned.

North Korea has built two tunnel entrances at its nuclear test site in a sign it plans more of its internationally condemned detonations, a U.S. think tank said Wednesday.

Satellite images taken on September 27 shows progress in excavation work and the presence of two new entrances to tunnels at the Punggye-ri site, Johns Hopkins University's U.S.-Korea Institute said.