A Pentagon spy agency has concluded with "moderate confidence" that North Korea has developed a nuclear-tipped ballistic missile, an assessment swiftly dismissed by several U.S. officials and South Korea, according to Reuters.

The Defense Intelligence Agency (DIA) said such a weapon would probably be unreliable. Its assessment, made public by a U.S. lawmaker in Washington, comes amid threats of war by North Korea and just hours before U.S. Secretary of State John Kerry arrives in Seoul on a visit to the region that will include stops in China and Japan.

South Korean and U.S. officials say Pyongyang appears set to test launch a medium-range missile as a show of strength ahead of the anniversary on Monday of the birth of North Korean founder Kim Il-Sung.

South Korea's Defense Ministry however said it did not believe North Korea could mount a nuclear warhead on a missile.

Despite the DIA report, the Pentagon's spokesman and the U.S. national intelligence director both said it was "inaccurate" to infer Pyongyang had the proven ability to launch a nuclear missile. The DIA was criticized after the start of the Iraq war in 2003 for being too bullish in predicting Baghdad might have weapons of mass destruction.

Its conclusion about North Korea follows more than a month of rising tension on the Korean peninsula.