North Korean leader Kim Jong-Un oversaw a live-fire artillery drill aimed at simulating an "actual war", state media said Tuesday, a day after South Korea swore in its first female president.

"An endless barrage of shells were fired by artillery pieces on 'enemy positions', their roar rocking heaven and earth, and all of them were enveloped in flames," the Korean Central News Agency (KCNA) said.

U.S. officials made two secret visits to North Korea last year in an effort to improve relations after North Korea leader Kim Jong Un assumed power, the Los Angeles Times reported Saturday.

Citing unnamed former U.S. officials familiar with the trips, the newspaper said the visits in April and August were aimed at encouraging the new leadership in Pyongyang to moderate its foreign policy.

Tokyo and Washington plan to install a U.S. early-warning radar system at a coastal base near Kyoto to bolster defenses against the North Korean missile threat, reports said Sunday.

The X-band radar, capable of precisely tracking the trajectory of a ballistic missile, allows U.S. forces to launch intercept missiles from the ground and sea once a ballistic missile has been detected.

North Korean leader Kim Jong-Un has overseen a military drill, state media said Saturday, his third such inspection in as many days as tensions run high following Pyongyang's third nuclear test.

Accompanied by top military commanders, Kim watched a flight exercise and a paratrooping drill by the Korean People's Army, the Korean Central News Agency (KCNA) said.

U.S. President Barack Obama on Friday pledged with Japan's new leader to take a firm line on a defiant North Korea but the two sides also tried to calm rising tensions between Tokyo and China.

Prime Minister Shinzo Abe carefully avoided disagreements with Obama after previous Japanese governments' rifts and declared: "The alliance between Japan and the United States is back now. It's completely back."

Japanese Premier Shinzo Abe headed to the White House Friday on a mission to send a strong signal to China, which has stepped up pressure on the conservative leader over a tense territorial row.

Two months after his right-leaning Liberal Democratic Party swept back into power, Abe opened his first meeting with President Barack Obama in hopes of showing the region that there is no daylight between the two allies.

North Korea will soon allow foreigners to tweet, Skype and surf the Internet from their cellphones, iPads and other mobile devices in its second relaxation of controls on communications in recent weeks. However, North Korean citizens will not have access to the mobile Internet service to be offered by provider Koryolink within the next week.

Koryolink, a joint venture between Korea Post & Telecommunications Corporation and Egypt's Orascom Telecom Media and Technology Holding SAE, informed foreign residents in Pyongyang on Friday that it will launch a third generation, or 3G, mobile Internet service no later than March 1.

Five North Koreans working on a building site in the Russian Far East have been found dead, apparently after inhaling fumes from a diesel-powered generator, investigators said on Monday.

The North Koreans were among the several thousand citizens of the secretive Stalinist state believed to be working around the city of Vladivostok in the Russian Far East, which has a small land border with North Korea.

North Korea has resumed activity at a nuclear site following its internationally condemned bomb test, a U.S. think tank said Wednesday, amid fears that the regime will carry out more explosions.

Examining satellite photos, the U.S.-Korea Institute at Johns Hopkins University detected a rise in traffic at the Punggye-ri site but cautioned that there was not enough evidence to assert that a new test was in the works.

North Korea is rewarding the scientists and workers behind its recent nuclear test with a fun-packed visit to Pyongyang to enjoy "the greatest privileges", state media said Wednesday.

Scientists, technicians, workers and officials behind the February 12 test received a hero's welcome when they arrived in the capital, the Korean Central News Agency (KCNA) said.