A conference held in Pyongyang ahead of the 90th founding anniversary of the People's Revolutionary Army, which falls April 25, is seen in this photo, released by North Korea's Korean Central News Agency, April 23. Yonhap

South Korea has detected signs of North Korea preparing to hold a massive military parade around midnight Sunday to show off its latest strategic weapons in time for a key anniversary, according to government sources.

The sources said the North was likely to mobilize around 20,000 troops for the parade at Kim Il Sung Square to mark the 90th founding anniversary of the North Korean People's Revolutionary Army (KPRA), that the North claims late national founder Kim Il-sung established in 1932.

During rehearsals, over 250 pieces of military equipment have been paraded, including a hypersonic Hwasong-8 missile, a Hwasong-17 intercontinental ballistic missile (ICBM) and a submarine-launched ballistic missile (SLBM), according to the sources.

The North also appears to have built two pontoon bridges across the Taedong River connecting Kim Il Sung Square to the Tower of the Juche Idea, raising possibilities that troops could use the bridges to enter the square amid fireworks so as to boost the festive mood.

The North has held nine military parades so far since leader Kim came to power in 2012 following the death of its late leader and his father Kim Jong-il, but none of them were held to mark the founding anniversary of the KPRA.

A performance held in Pyongyang ahead of the 90th founding anniversary of the People's Revolutionary Army, which falls April 25, is seen in this photo released by North Korea's Korean Central News Agency, April 23. Yonhap

The North has usually held such a parade on the anniversary of the birth of national founder Kim Il-sung, April 15, the founding anniversary of the ruling Workers' Party, Oct. 10, and the nation's foundation, Sept. 9.

Observers say the North appears to have decided to hold the parade to mark the day its "anti-Japanese guerrilla forces" were created during Japan's colonial rule, in an apparent effort to rally internal support and show off its strategic weapons.

Kim could deliver a speech during the parade to send a message to both his people and the outside world.

The military parade also comes ahead of South Korean President-elect Yoon Suk-yeol's inauguration next month. Yoon is expected to take a hardline stance on North Korea.

Pyongyang is likely to broadcast recorded footage of the parade on state television Monday. (Yonhap)