"Signs of a military parade -- involving equipment and people on a large scale -- were detected at Kim Il Sung Square early this morning," Seoul's joint chiefs of staff said in a statement.

South Korean and US intelligence agencies were "closely tracking the event", they added.

The widely anticipated parade is part of commemorations of the 75th anniversary of the North's ruling Workers' Party.

Such events typically feature thousands of goose-stepping troops followed by a cavalcade of larger armoured vehicles and tanks, culminating with whatever missiles Pyongyang wants to display.

They are closely watched by observers for clues to its weapons development.

By early afternoon, state broadcaster KCTV had yet to show any images of a parade.

But specialist service NK News cited multiple sources saying they had heard sounds of aircraft flying by, drones and heavy machinery during the early hours of Saturday morning.