Seoul, South Korea - US and South Korean troops have begun large-scale spring military exercises as tensions with North Korea mount.

The annual training exercise, codenamed Freedom Shield, started as planned, the Ministry of National Defense in Seoul said on Monday.

The 11-day exercise includes computer simulations and drills in the field. It was initially unclear how many soldiers would be involved.

A small group of anti-war activists protested in Seoul on Monday, demanding an end to the yearly event.

According to South Korea, defense against nuclear threats from North Korea will be a core element of the training exercise.

The US and North Korea's neighbors South Korea and Japan see themselves directly threatened by Pyongyang's nuclear weapons and missile programs. The isolated one-party state in the north, ruled by an increasingly belligerent Kim Jong-un, sees Washington as particularly hostile.

Freedom Shield is described as a multidimensional defense training program that includes cyber and information space as well as outer space as possible battlefields in addition to land, sea, and air.

According to the US Forces Korea (USFK), military personnel from 10 member countries of the US-led UN Command (UNC), which monitors compliance with the 1953 armistice agreement, are also taking part.

Pyongyang regularly accuses Washington and Seoul of preparing for an attack with their military exercises, which both countries deny.