SEOUL, March 14 -- South Korea's military on Monday refused confirmation on a Democratic People's Republic of Korea (DPRK) submarine, which United States media reported had gone missing for days.

CNN reported Saturday citing U.S. officials as saying that one of DPRK submarines operating off its east coast went missing earlier last week.

According to the media report, the DPRK navy searched for the missing sub, which U.S. spy satellites, aircraft and ships had also been secretly watching.

The U.S. military believed that the DPRK sub suffered some types of failure during an exercise.

Seoul's Defense Ministry spokesman Moon Sang-Kyun told a regular press briefing that the intelligence authorities of both South Korea and the United States maintain a position that they cannot confirm the relevant report.

The report came amid heightened tensions on the Korean Peninsula after Seoul and Washington launched their largest-ever joint annual war games, code-named Key Resolve and Foal Eagle, last Monday.

The drills, which Pyongyang has denounced as a dress rehearsal for northward invasion, would last until April 30.

The DPRK repeated its warnings against the spring exercises on Sunday, threatening a "pre-emptive and offensive nuclear strike" toward South Korea and the U.S. mainland.

Before the launch of the drills, top DPRK leader Kim Jong Un ordered nuclear warheads to be placed always on standby for use at any time.

Kim said on Wednesday that his country has succeeded in miniaturizing nuclear warheads to fit on ballistic missiles at a meeting with nuclear scientists and technicians.

Regarding this, Moon reiterated Seoul's position that Pyongyang has advanced its nuclear miniaturization technology to a significant level but has yet to secure a technology to miniaturize nuclear warheads.

He said U.S. and South Korean intelligence authorities are precisely analyzing whether the DPRK secured nuclear warheads small enough to be mounted on ballistic missiles.