SEOUL, Dec 25 (Reuters) — North Korean leader Kim Jong Un recently inspected the construction of a submarine alongside his daughter, who is seen as a possible heir. He also oversaw a test-firing of long-range surface-to-air missiles, according to state media reports.

The missile test, conducted near the east coast, aimed to evaluate North Korea's technology for developing a new type of high-altitude missile. The missiles successfully hit targets located 200 km (120 miles) away, as stated by KCNA.

South Korea's Joint Chiefs of Staff confirmed the missile test took place at 5 p.m. (0800 GMT). The South Korean military was aware of the launch preparations and was ready for the event, reported Yonhap News Agency.

Kim also checked on the construction of an 8,700-ton nuclear-powered submarine, which is designed to launch surface-to-air missiles. KCNA did not disclose the specific location or date of his visit.

This submarine project is part of North Korea's ruling party's efforts to modernize its navy, which is one of five key military development policies being promoted by the regime.

A photograph from KCNA showed Kim at the submarine construction site with his daughter, Ju Ae. Surrounded by officials, Kim smiled while the teenager, viewed by some analysts as a likely successor, stood beside him in a facility housing a red submarine.

Kim announced that North Korea is constructing multiple attack destroyers and nuclear submarines, and is seeking to speed up construction so these vessels can be armed with various weaponry, according to KCNA.

A senior research fellow at the Korea Institute for National Unification in Seoul, Hong Min, said in a report that the submarine's hull design suggests it is equipped with a nuclear reactor and is close to being ready for operation.

Kim emphasized that developing nuclear capabilities and modernizing the navy are crucial and unavoidable, stating that "the present world is by no means peaceful."

He also warned that South Korea's plans to develop a nuclear-powered submarine, in agreement with the U.S., would heighten tensions on the Korean Peninsula and threaten national security, prompting the need for North Korea to take action.

Additionally, North Korean state media criticized the recent docking of a U.S. nuclear-powered submarine in South Korea, labeling it "an act of escalating military tensions" in the region.

On Tuesday, the nuclear-powered submarine Greeneville arrived at Busan port for crew rest and supply loading, according to the South Korean navy.

This week, North Korea accused Japan of showing intentions to acquire nuclear weapons, influenced by South Korea's submarine development efforts.