SEOUL, Oct. 7 (UPI) -- New activity is visible at North Korea's Punggye-ri nuclear site, where Pyongyang conducted its fifth nuclear test in September.
Writing for 38 North, a Johns Hopkins University website dedicated to North Korea issues, analyst Jack Liu stated on Thursday satellite imagery from Oct. 1 show "continuing activity" at all three tunnels on site.
Activity at the North Portal, the site of the last test, could be taking place for a number of reasons: data collection, shutdown or preparations for a sixth test, according to Liu.
There is also activity at the South Portal and several carts of vehicles at the West Portal.
U.S. and South Korean intelligence have picked up on other activity at the Sohae Satellite Launching Station in Tongchang-ri, North Pyongan province, Yonhap reported Friday.
The site where Pyongyang launched a long-range rocket to send a satellite into orbit in February is the center of a "visible increase" in activity, a senior South Korean official said.
"It appears that they are making preparations, possibly for a long-range missile launch, or another strategic provocation," the official said.
North Korea had stated in mid-September that it now has a new line of rockets, the "Paektu series" of long-range missiles, raising the possibility a future launch would involve one of the new models.
South Korea's military is also monitoring any North Korea movements. Pyongyang is expected to commemorate an anniversary of the founding of the Korean Workers' Party on Monday, and Seoul's armed forces are on emergency standby, according to Yonhap.
North Korea has increased its verbal attacks of the South since Seoul's defense minister confirmed a plan known as Korea Massive Punishment and Retaliation, an attack that could target Kim Jong Un in the event of a North Korea nuclear assault.
Joint chiefs of staff chairman Gen. Lee Sun-jin said Friday the plan does not entail a pre-emptive strike, but rather a response in the case of an attack, according to News 1, repudiating reports that claimed the plan includes a pre-emptive strike on the North Korean leadership.