The U.S. government on Thursday admitted hackers accessed the personal data of at least four million current and former federal employees, in a vast cyber-attack suspected to have originated in China.

"As a result of the incident," uncovered in April, the Office of Personnel Management said it "will send notifications to approximately four million individuals."

South Korea on Wednesday test-fired two ballistic missiles capable of hitting any part of North Korea, in an apparent response to recent bellicose boasts from Pyongyang about its nuclear and missile capabilities.

In an unusual move, President Park Geun-Hye personally observed the test exercise, a defense ministry official said.

South Korea and the United States kicked off a major anti-submarine drill on Monday, weeks after North Korea claimed it had successfully tested a submarine-launched ballistic missile.

The three-day exercise, aimed at tackling provocations by North Korean submarines, is being conducted off the southern island of Jeju, the South's defense ministry said.

An exiled Iranian opposition group accused Tehran Thursday of a "vast collaboration" with North Korea in developing nuclear arms, alleging that experts from both countries made regular intelligence-sharing visits.

"The Iranian regime continues to collaborate with North Korea on nuclear warheads and ballistic missiles," the National Council of Resistance of Iran (NCRI) said in a report citing sources close to the Iranian government.

Fresh satellite images show substantial new construction at North Korea's space rocket launch site, mirroring leader Kim Jong-Un's recent vow to launch more satellites in defiance of U.N. resolutions, a U.S. think-tank said Friday.

The impoverished but nuclear-armed North sent an Unha-3 rocket into space in December 2012 from the Sohae satellite launch site -- a move the international community condemned as a disguised ballistic missile test and punished with further sanctions.

Nuclear envoys from South Korea, Japan and the United States agreed Wednesday to step up pressure -- including possible sanctions -- on North Korea to abandon nuclear weapons, while keeping diplomatic options "open."

Speaking to reporters after trilateral talks in Seoul, the three officials also highlighted the "seriousness" of the progress the North has been making with its expanding nuclear and ballistic missile programs.

North Korean leader Kim Jong-Un has hailed the recent test of a submarine-launched ballistic missile (SLBM) -- which was viewed with wary skepticism outside the reclusive state -- as a miraculous leap forward.

The North's official KCNA news agency reported Tuesday that Kim hosted a gathering of the scientists and technicians who were behind the test that was announced with great fanfare earlier this month.

An international group of women peace activists, led by American feminist Gloria Steinem, made a rare crossing Sunday of one of the world's most militarized borders between North and South Korea.

The group of 30 activists rode by bus through the demilitarized zone (DMZ) separating the two Koreas in what Steinem described as a "triumph" for peace and reconciliation, dismissing criticism that the women had allowed themselves to be used as propaganda tools by the North.

A group of international women peace activists broadcast a landmark press conference from North Korea on Saturday, using Twitter's new video live-streaming app Periscope, apparently with official approval.

The group of 30 women, led by American feminist Gloria Steinem, are scheduled to cross the demilitarised zone (DMZ) dividing North and South Korea on Sunday to promote peace and reconciliation.

China called for restraint Thursday after North Korea claimed to have developed the ability to deliver miniaturized warheads on high-precision long range rockets.

"China urges parties concerned to exercise restraint, avoid provoking each other, (and) do more to ease tension," foreign ministry spokesman Hong Lei told reporters at a regular briefing in Beijing.