North Korea's leader Kim Jong-un called for the country's military to “bolster up their strength in every way to annihilate the enemy”, state media reported on Friday.
Mr Kim made the remarks during photo sessions with troops, state media broadcasters, and others involved in a huge military parade on Monday, which marked the 90th anniversary of the army's founding.
The latest satellite imagery showed increased preparations for a possible nuclear test.
“Current satellite imagery indicates that preparations are well under way and should not be discounted as insignificant activity,” the Centre for Strategic and International Studies (CSIS) said in a report on Thursday.
Commercial satellite imagery from Monday shows construction of new buildings, movement of lumber, and an increase in equipment and supplies immediately outside the new entrance to tunnel number three, CSIS said.
“The date of a seventh nuclear test will undoubtedly depend exclusively upon the personal decision of Kim Jong-un,” the report said.
During the parade, the North Korean leader promised to speed up development of its nuclear arsenal.
The parade featured several of the North's latest missiles, including its largest intercontinental ballistic missile, the Hwasong-17, and a recently tested hypersonic missile.
North Korea displays ICBMs at huge parade
The display demonstrated the “modernity, heroism and radical development of the armed forces of the republic and their matchless military and technological superiority”, Mr Kim told troops at the photo session, state news agency KCNA reported.
North Korea has stepped up weapons tests and displays of military power in recent months, with US and South Korean officials saying there are signs of new construction at North Korea's only known nuclear test site.
The site has been officially closed since 2018, Reuters reported, suggesting Pyongyang may be preparing to resume testing nuclear weapons.
Meanwhile, two South Koreans have been arrested on charges of stealing military secrets for a suspected North Korean agent who paid them in cryptocurrency, Seoul police said on Friday.
One of them is a 29-year-old army captain who allegedly passed log-in information for South Korea's Joint Command & Control System, to the suspected Pyongyang spy, Seoul authorities said.
The other, a 38-year-old businessman who runs a virtual asset management company, is accused of giving the army captain a wristwatch with a secret camera to aid in intelligence gathering, at the request of the spy.
The businessman is also suspected of buying and assembling a USB-like hacking device called “Poison Tap” to access the Joint Command & Control System, Seoul police said.
“The two men have been arrested on charges of violating the national security law,” an official at the Korean National Police Agency told AFP on Friday.
Both were paid in cryptocurrency, the police said. The army captain received about 48 million won ($37,789) from the North Korean agent, while the businessman received about $600,000.
“We will strictly respond to security criminals in accordance to laws and principles,” Seoul's prosecution office said.
North and South Korea remain technically at war because the 1950-1953 Korean War ended with the signing of an armistice and not a peace treaty.
Agencies contributed to the report
