The news comes as South Korea makes its own display of military force and as the US accuses North Korea of “begging for war”.
North Korea has been seen moving what appears to be an intercontinental ballistic missile towards its west coast, a report says.
South Korea’s Asia Business Daily cited an intelligence source as saying that the rocket was spotted moving on Monday, the day after North Korea’s sixth and largest nuclear test.
The North Korean missile programme’s launch facilities are on its west coast.
The move was made during the night to avoid surveillance, according to the report.
South Korea’s defence ministry said it could not confirm the report but on Monday it said that the North was ready to launch more missiles.
In July, North Korea tested two ICBMs capable of flying about 6,200 miles, threatening parts of the US mainland.
On Tuesday local time, the South Korean Navy held live-fire drills in the Sea of Japan.
These involved the 2,500-tonne frigate Gangwon, a 1,000-tonne patrol ship and 400-tonne guided missile vessels in a show of force that aimed to deter the North.
Captain Choi Young-Chan, commander of the 13th Maritime Battle Group, said: “If the enemy launches a provocation above water or under water, we will immediately hit back to bury them at sea.”
It was agreed on Monday that the country could scrap a warhead weight limit on its missiles, meaning it could hit back at its northern neighbour with greater force.
North Korea alarmed the region with its most powerful test to date on Sunday of what it claimed was a hydrogen bomb able to be mounted onto a long-range missile.
The US responded by saying that North Korea leader Kim Jong Un was “begging for war”.
The American ambassador to the UN, Nikki Haley, said her country did not want a conflict with the Pyongyang regime but its patience was “not unlimited”.
Speaking at the UN Security Council, Ms Haley said “enough is enough”, warning the organisation that its approach of imposing “incremental” sanctions against North Korea had not worked.
Sunday’s device is thought to have been about five times larger than the bomb dropped on the Japanese city of Nagasaki in World War II and it caused a tremor with a magnitude of 6.3.