South Korea says it has started to check for radiation in the atmosphere at a military facility along its border with the DPRK.

This follows Pyongyang's second nuclear test. And the US and Japan have also started to gather evidence relating to the test.

A US Air Force jet, believed to be a reconnaissance aircraft capable of detecting radiation in the atmosphere, took off from an air base in Japan on Tuesday.

The aircraft can collect and analyze radioactive substances in the air. And Japan says it is also monitoring radioactivity in the air.

US intelligence authorities confirmed the DPRK's first nuclear test in 2006 after analyzing radioactive substances from samples.

(CCTV May 27, 2009)