South Korea and the United States began three days of military drills Monday off the coast of the South Korean island of Jeju. The anti-submarine drills began just weeks after North Korea claimed to have tested a submarine-launched ballistic missile (SLBM).

“This is the largest joint anti-submarine exercise the allies have ever staged, in terms of its scale and the number of vessels involved,” Press TV quoted an anonymous Defense Ministry spokesman as saying, as Voice of America reported. The official added that the naval drills would include helicopters, submarines, surveillance aircraft and a South Korean Aegis destroyer.

North Korea has been skeptical about the joint military efforts by South Korea and the United States. It calls them the principal reason behind the strained relations in the region. North Korea, on the other hand, claimed it had acquired the ability to develop a miniaturized nuclear warhead.

The South Korean ministry said there was a separate joint live-fire exercise with its naval vessels, artillery units and warplanes. The exercise was conducted off the east coast of Goseong County, south of the border with North Korea.

“The drill is aimed at deterring submarine-related provocations and beefing up the navy’s capacity to protect a southern maritime pathway, a key factor in maintaining operational logistics during war,” the Korea Observer quoted an unnamed official as saying. According to Washington, North Korea’s nuclear program breaches international standards.

The Star Online reported that U.S. authorities tried deploying the Stuxnet computer virus against North Korea's nuclear weapons program five years back. However, the effort failed. It apparently sabotaged Iran's nuclear program in 2009 and 2010.

A former U.S. official said that that North Korea’ tight secrecy had been responsible for the failure of the National Security Agency-led campaign. In North Korea, it takes police permission to own a computer. The Internet is highly scrutinized in the country as well.