North Korea is building a new complex at its main rocket launch site, possibly for training and launches of road-mobile intercontinental ballistic missiles, a U.S. think-tank said.

Satellite imagery from May 10 suggests the North is conducting a number of important construction projects at the Sohae Satellite Launching Station on its western coast, the U.S.-Korea Institute at Johns Hopkins University said Tuesday.

A South Korean naval ship fired warning shots Tuesday after three North Korean patrol boats crossed over the disputed maritime border in the Yellow Sea, military officials said.

The North Korean boats, which normally serve to keep fishing boats on the right side of the boundary, crossed into South Korean waters at 0700 GMT, the South's joint chiefs of staff said.

South Korea offered its condolences to North Korea on Tuesday over the deadly collapse of a Pyongyang apartment building that led to a rare apology from the secretive hardline state.

The Koreas have been locked in an escalating war or words lately over the origin of spy drones but both have still managed to send messages of condolence at times of tragedy.

Japan said Monday it will hold talks with North Korea next week in Sweden, seeking to speed up discussions on the Japanese nationals kidnapped by North Korean spies decades ago.

Japanese Foreign Minister Fumio Kishida told reporters in Tokyo that the government-level talks will be held on May 26-28 in Stockholm.

South Korean President Park Geun-Hye took tearful responsibility Monday for the mishandling of last month's Sewol ferry disaster, admitting many lives were unnecessarily lost and vowing to dismantle the national coastguard.

"The ultimate responsibility for the poor response to this accident lies with me," Park said in a televised address to the nation, during which she openly wept and twice bowed deeply in a display of contrition.

North Korea's state media reported Sunday an "unimaginable" accident at an apartment construction site in Pyongyang, which had resulted in an unspecified number of casualties.

South Korean officials said the incident involved the collapse of a 23-storey apartment building, which already had close to 100 families in residence.

Satellite images have picked out two new North Korean warships -- the largest it has constructed in 25 years and an important "wake-up call" on the effectiveness of sanctions, a U.S. think-tank said Friday.

Recent commercial satellite pictures showed two new helicopter-carrying frigates separately berthed at shipyards in Nampo in the west and Najin in the far northeast.

Iran is sharing nuclear technology with North Korea, Israeli Prime Minister Benjamin Netanyahu said in an interview published Wednesday, as Tehran and world powers hold talks aimed at ending a decade-old standoff.

Netanyahu, who is in Japan this week for talks with Prime Minister Shinzo Abe and Foreign Minister Fumio Kishida, said Iran "would share whatever technology it acquired with North Korea," the Mainichi Shimbun reported in a front-page piece.

South Korea's defense ministry said Wednesday that a suspected crashed drone spotted by a walker was a false alarm -- it was actually the door of a portable toilet.

A passer-by had reported seeing what he thought was the wreckage of a drone caught among rocks and trees in a fenced off military area, while walking on a hillside in southern Seoul.

Despite fears to the contrary, North Korea does not appear to be preparing an imminent nuclear test, a U.S. think-tank said Wednesday in an analysis of recent satellite images of Pyongyang's main test site.

While the latest pictures dated May 9 do show high levels of activity at the Punggye-ri site, most of it seems to be of a mundane, routine nature that would not be consistent with an impending test, the U.S.-Korea Institute at Johns Hopkins University said.