A U.N. investigator on Friday ruled out any compromise on efforts to have North Korea answer formal charges of crimes against humanity, saying holding the regime to account, including supreme leader Kim Jong-Un, is "paramount".

Marzuki Darusman, the U.N.'s special rapporteur on human rights in the North, also stressed that Kim's refusal to halt human rights atrocities instigated by previous regimes would make him legally "complicit" in such crimes.

Under growing pressure at the United Nations over its human rights record, North Korea has accused rival South Korea of its own "crimes against humanity" over the Sewol ferry tragedy.

Just hours after a South Korean court on Tuesday sentenced the captain of the Sewol to 36 years, the North issued a report "indicting" the South for "hideous crimes" and rights abuses in its handling of the disaster that killed more than 300 people.

A secret mission by America's spy chief to free two U.S. citizens held by North Korea hit an embarrassing snag when his plane broke down while refueling, a U.S. official said Monday.

Director of National Intelligence James Clapper was sent to Pyongyang last week as the personal envoy of President Barack Obama to bring back detainees Kenneth Bae and Matthew Miller.

President Barack Obama on Monday dismissed North Korea's release of two imprisoned Americans as "small gestures", saying nuclear-armed Pyongyang must change its attitude on atomic weapons if it wants improved ties.

"We have been consistent in saying that when and if North Korea becomes serious about de-nuclearization on the peninsula and is prepared to have a conversation around that topic, then the United States is going to be very open to try to arrive at a solution," Obama told reporters in Beijing.

South Korean troops fired warning shots Monday as a North Korean patrol approached the heavily fortified border inside the so-called Demilitarized Zone (DMZ) that separates the two rivals.

There was no exchange of fire and the North Korean patrol eventually moved away from the military demarcation line, a defense ministry spokesman said.

U.S. intelligence chief James Clapper made a secret mission to North Korea to secure the release of two Americans at Pyongyang's initiative, but did not meet leader Kim Jong-Un, an official said.

Clapper, who flew back with the two freed men to an airbase in Washington state on Saturday, conducted talks with senior North Koreans but stayed less than a day and never met Kim, a U.S. senior administration official said.

North Korea's surprise release of two jailed American citizens is further evidence that Pyongyang is genuinely rattled by moves at the United Nations to charge its leadership with crimes against humanity, analysts said Sunday.

The release of Kenneth Bae and Matthew Miller came days after an EU-Japan draft resolution -- co-sponsored by 48 countries -- was submitted to a U.N. General Assembly committee, urging the Security Council to refer the Pyongyang regime to the International Criminal Court (ICC).

Two Americans released from lengthy prison sentences in North Korea arrived home Saturday, with one thanking supporters for never giving up on him.

The plane carrying Kenneth Bae and Matthew Miller touched down at around 9:00 pm (0500 GMT) at Joint Base Lewis-McChord in Washington State, following a secret mission by U.S. intelligence chief James Clapper to secure their freedom.

Two Americans serving lengthy prison sentences in North Korea were released on Saturday by the reclusive state following a secret mission to Pyongyang by U.S. intelligence chief James Clapper, the State Department announced.

Kenneth Bae and Matthew Miller were the last two American nationals held by North Korea following the sudden release last month of 56-year-old Jeffrey Fowle.

North Korean state TV has aired footage of Kim Jong-Un for the first time in more than two months, showing the leader walking without a stick but with a pronounced limp.

The video footage, aired late Thursday by the North's Chosun Chungang TV, showed Kim on stage at a meeting of military commanders in Pyongyang that took place on Monday and Tuesday.