North Korea readied Thursday to kick off its most important ruling party gathering for nearly 40 years, amid persistent concerns of a nuclear test, despite no clear signs of an imminent detonation.

Leader Kim Jong-Un is expected to deliver a keynote address at the opening of Friday's party congress which will be minutely scrutinized for suggestions of a significant policy shift or personnel changes in the nuclear-armed nation's governing elite.

After four years of top-level reshuffles, purges and executions, Kim Jong-Un will formally cement his unassailable status as North Korea's supreme leader at a landmark ruling party congress this week.

The first gathering of its kind for nearly 40 years is really a coronation of sorts -- recognizing the young 33-year-old leader as the legitimate inheritor of the dynastic dictatorship started by his grandfather Kim Il-Sung and passed down through his late father Kim Jong-Il.

North Korea on Saturday tested what appeared to be a submarine-launched ballistic missile in the Sea of Japan, the South Korean defense ministry said.

"North Korea launched a projectile which was believed to be a submarine-launched ballistic missile (SLBM) around 6:30 pm (0930 GMT) in the East Sea (Sea of Japan) near the northeastern port of Sinpo", a defense ministry spokesman said.

South Korea, Japan and the United States on Tuesday warned North Korea of harsher sanctions and deeper isolation if it went ahead with a fifth nuclear test or other provocations.

The warning, which followed a trilateral meeting of top diplomatic officials, came amid growing speculation that Pyongyang is in the final stages of preparing an underground nuclear detonation at its Punggye-ri test site.

A surge in activity at North Korea's atomic test site suggests preparations for a fifth nuclear test are in their final stages, possibly before a key political event in early May, South Korean media reported Sunday.

The frequency of vehicle, workforce and equipment movements at the Punggye-ri site have "increased two to threefold," since last month, Yonhap news agency said, citing multiple government sources.

Pyongyang's state media has published an open letter purportedly from Abraham Lincoln to President Barack Obama, chastising him for stifling the "government of the people, by the people, for the people" -- ie, North Korea.

In the piece uploaded on the North's propaganda website "DPRK Today" on Monday, the 16th U.S. president who was assassinated in 1865 "advised" the 44th president that North Korea would never crumble under sanctions and economic blockades.

North Korea accused South Korea Tuesday of kidnapping its citizens after Seoul said 13 of them had defected to the South from China, where they worked in a Pyongyang-operated restaurant.

In its first reaction since Seoul announced the defections, the North's Red Cross spokesman accused the South of committing a crime on an "unparalleled" scale by "kidnapping" them.

North Korea said Saturday it had successfully tested an engine designed for an inter-continental ballistic missile (ICBM) that would "guarantee" an eventual nuclear strike on the US mainland.

It was the latest in a series of claims by Pyongyang of significant breakthroughs in both its nuclear weapons and ballistic missile programs.

Thirteen North Koreans working in a state-run restaurant outside the country have defected to South Korea, a government official in Seoul said Friday.

The South Korean government estimates that Pyongyang rakes in around $10 million every year from some 130 restaurants it operates -- with mostly North Korean staff -- in 12 countries, including neighboring China.

North Korea's top military body has accused U.S.-led "hostile forces" of laying siege to the country like Leningrad in World War II and Cuba during the Cold War missile crisis.

In a statement carried Monday by the North's official KCNA news agency, a spokesman for the National Defense Commission (NDC) also said the latest UN sanctions imposed on Pyongyang over its nuclear weapons program were "anachronistic and suicidal" and could trigger a nuclear strike on the U.S. mainland.