Satellite images expose "ongoing activity" at a North Korean shipyard used to equip submarines with ballistic missiles, analysts say.
Commercial photographs captured above the Sinpo South Shipyard have raised fears Kim Jong-un is working to develop a new range of submarine-launched weapons.
Experts say the aerial images show experimental work on a ballistic missile submarine has been completed after netting previously seen at the site was removed.
The pictures show a barge which is believed could be used to launch an submarine-launched ballistic missile (SLBM) test has remained in the same position where it was observed last month, according to the 38 North Blog.
North Korean analyst Joseph S. Bermudez Jr said an unknown "shipbuilding program is probably underway".
But the expert said the activity did not reflect an "imminent test" despite "growing concern" over feared plans for a submarine-launched ballistic missile (SLBM) drill.
Writing on the US-Korea Institute at Johns Hopkins University's 38 North blog, Joseph S. Bermudez Jr told the pictures were taken from September 21.
He said: "While there is growing concern that the North may be planning a submarine-launched ballistic missile (SLBM) test in the near future, the current imagery does not indicate an imminent test.
"The SINPO-class submarine, submersible test stand barge and nearby test stand, however, appear capable of supporting a test at any time of Pyongyang’s choosing."
In August, military chiefs raised fears North Korea was looking to upgrade its submarine launch system Pukkuksong-1 when Kim was pictured next to designs showing new warheads.
The revelation comes the day after US flew two bomber jets over the Korean peninsula despite threats from the Kim Jong-un regime that they will be shot down.
On Tuesday, six military planes, including two American supersonic B-1B bombers, took part in a drill over the Korean peninsula, according to Yonhap news agency.
The bombers from Guam staged a simulated air-to-ground missile firing drill with two F-15K fighters, according to senior US defence officials.
It was the latest provocation amid heightened tensions between the US and North Korea.
Last month, foreign minister Ri Yong Ho accused the United States of having declared war.
In a rare press appearance at the United Nations in New York, he said: "The whole world should clearly remember it was the US who first declared war on our country.
"Since the United States declared war on our country, we will have every right to make countermeasures, including the right to shoot down United States strategic bombers even when they are not inside the airspace border of our country."
Earlier it emerged North Korean hackers are feared to have stolen a huge haul of classified military documents - including detailed plans revealing what the US and South Korea would do if the countries went to war.
The admission came by a senior lawmaker in South Korea.
Among the plans which Kim Jong-un's regime may now have its hands on are details on key military facilities and contingency plans for the South's special forces.
Hackers are said to have broken into the Defence Integrated Data Center last year, and several secret files are thought to have been taken.
A statement from Democratic Party Rep Lee Cheol-hee said, Yonhap News reports: "The Ministry of National Defense has yet to find out about the content of 182 gigabytes of the total (stolen) data."
He said 235GB had been taken, and 80 per cent of these have yet to be identified.
Pyongyang has denied responsibility for the cyberattacks, Yonhap reported, criticising Seoul for "fabricating" claims about online attacks.
It comes as it emerged that US President Donald Trump may visit the heavily fortified demilitarised zone (DMZ) separating North and South Korea when he visits South Korea next month.
A defence source told Yonhap that the village of Panmunjom and an observation post, both inside the DMZ, were among locations Trump was considering visiting.
The White House has yet to comment on the report.
Hostilities between Trump and North Korean leader Kim Jong-un have escalated in recent weeks, with the US President describing his counterpart as 'Rocket Man' to the UN.
Trump has suggested the military option was the only way to halt the North's missile and nuclear programmes.
North Korea is likely to view a visit to the DMZ by Trump to be highly provocative.
In recent weeks, North Korea has launched two missiles over Japan and conducted its sixth nuclear test, all in defiance of U.N. Security Council resolutions, and may be fast advancing toward its well publicised goal of developing a nuclear-tipped missile capable of hitting the U.S. mainland.
Last week Trump dismissed the idea of talks with North Korea as a waste of time, a day after Secretary of State Rex Tillerson said Washington was maintaining open lines of communication with Kim Jong-un's government.
"Our country has been unsuccessfully dealing with North Korea for 25 years, giving billions of dollars & getting nothing. Policy didn't work!" the U.S. president said in a Twitter post on Monday.
Reclusive North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.
Trump is scheduled to visit Japan, South Korea, China, Vietnam and the Philippines starting from Nov. 3.