North Korea appears to have held a military parade early on Saturday to mark the 75th anniversary of the founding of its ruling Workers' Party, South Korea's military said.
The anniversary, celebrated with a raft of concerts and festivals, was closely watched around the region as it was seen as an event where leader Kim Jong Un could deliver messages to domestic and foreign audiences.
The North's state media has not shown any images of a parade.
South Korea's Joint Chiefs of Staff said it spotted signs that a parade involving large-scale military equipment and personnel took place in the capital Pyongyang but gave no further details.
"There was a sign that North Korea conducted a military parade this morning at Kim Il Sung Square, mobilising large scale equipment and personnel," it said in a statement.
"South Korea and U.S. intelligence authorities are closely monitoring developments, including for the possibility that it was the main event."
For weeks commercial satellite imagery has shown thousands of North Korean soldiers practicing marching, and South Korean officials have said the North could use a parade to unveil a new intercontinental ballistic missile (ICBM), or a new submarine-launched ballistic missile.
Foreign diplomats in Pyongyang have often been invited to observe past celebrations. But the Russian Embassy said on social media that all diplomatic missions have been advised this year to "refrain as much as possible" from travelling in the city, approaching the event venue and taking photos and videos.
The event comes as the isolated country carries out strict measures to prevent the new coronavirus - the secretive state has not reported any domestic infections. State media said the curbs have caused delays in some of Kim's key economic and construction projects, already dogged by international sanctions.
"But it is an impressively large gathering during a global pandemic, suggesting North Korean authorities are concerned more with political history and national morale than with preventing a COVID-19 superspreader event," said Leif-Eric Easley, who teaches at Ewha Womans University in Seoul.
The last time North Korea broadcast a military parade live on television was in 2017, when it showed off many large ICBMs amid heightened tension with the United States.
ICBMs were once again paraded in February 2018, but no international media were allowed to observe. Shortly after, Kim began meeting international leaders such as U.S. President Donald Trump, and no large missiles have been displayed since.
Denuclearisation talks with Washington have broken down, and South Korean officials said on Thursday that Kim could use the military parade as a "low intensity" show of power ahead of the U.S. presidential election in November.
In a congratulatory message to Kim for the anniversary, Chinese President Xi Jinping said he intended to "defend, consolidate and develop" ties with North Korea, its state media said on Saturday.