Kim Jong-un could unveil new weapons in the coming weeks, North Korea experts believe.
Suspicions have mounted as troops have been practising for a military parade to celebrate the anniversary of the ruling Workers' Party next month.
Some observers say North Korea may use the Pyongyang parade to showcase its largest missiles for the first time since 2018.
Satellite photos have also shown a new "flurry" of activity at the Sinpo South Shipyard weapons base where submarines are built.
"We're monitoring developments, as there is a possibility that a submarine-launched ballistic missile test will be conducted there," said South Korea's incoming military chief Won In-choul.
Experts say the push from the North to acquire submarine-launched missiles is a worrying development because they are difficult to detect before they are launched.
A source told the Seoul-based Daily NK website that the shipyard “is full of activities to prepare for the ballistic missile launch".
Dave Schmerler, a researcher at the James Martin Center for Nonproliferation Studies, said there were signs of work being done on the missile barge.
On September 4, the US-based Center for Strategic and International Studies said satellite images showed activity in Sinpo that was “suggestive, but inconclusive, of preparations for an upcoming ballistic missile test".
Seoul's unification minister Lee In-young said this week: "I hope that communication channels, including the inter-Korean liaison office, can be restored and we can resume open-minded dialogue at an early date."
Jung Changwook, head of the Korea Defense Study Forum in Seoul, said North Korea could perform a test to upgrade its nuclear attack capability and put pressure on Washington after the US election in early November.
Jung said a missile fired from the barge would have a potential range of up to 310 miles, posing no direct threat to the US mainland.
It comes after Kim Jong Un made a rare public appearance last week after five government officials who spoke out against the regime's economic policies were reportedly executed.
The North Korean leader inspected reconstruction work in a flood-hit area in Unpha County, North Hwanghae Province, state media KCNA reported.
A series of typhoons and the wettest monsoon battered several parts of the country in recent weeks.
Kim said the North Korean economy “has faced trouble and distress at the great damage caused by the recent series of heavy rain and typhoons,” according to KCNA.
