South Korea and the US have agreed to step up their surveillance of underwater activities in the Korean peninsula to closely monitor the movements of North Korea's submarines. The move follows Pyongyang firing a submarine-launched ballistic missile (SLBM) adding to the tensions in the region.

Seoul and Washington have agreed to share more information on the underwater activities in case the North intensifies its missile programme. The surveillance will also focus on the seas surrounding Pyongyang's main submarine base in Sinpo, located in the country's northeastern province of Hamkyung. The port hosts Pyongyang's Gorae-class ballistic missile submarine programme.

The focus of the ongoing two-week-long military exercises of South Korea and the US has also been recalibrated to deal with the situation as several anti-submarine acts have been included.

"The underwater environment related to the military operations comprises topographical features, water temperature, depth of water and tidal currents, among others. A thorough analysis of them will help detect North Korean submarines' moves and possible infiltrations into the South's seawaters," a South Korean official told Yonhap news agency.

The North blasted off an SLBM on 24 August as an act of defiance against its adversaries, the South and the US. The missile flew up to 500kms before ending up in the East Sea or the Sea of Japan.

One of the key areas where Washington and its ally would increase surveillance would be the Yellow Sea near the disputed border known as Northern Limit Line (NLL) to look for any possible infiltration by the North. The US navy's nuclear-powered underwater vessels are already known to keep a close watch on the region, sources in Seoul's defence ministry told Yonhap.

This is in addition to the South Korean navy placing its forces on alert to look for any sea-based infiltration from North Korea. The South Korean navy's chief, Admiral Jung Ho-sub, has ordered Seoul's forces to be in full combat readiness to counter any provocation from the North.