SEOUL--North Korea has begun tests to load anthrax onto intercontinental ballistic missiles, according to an intelligence source here.

The United States also has come to the same conclusion. The White House, in its new National Security Strategy issued Dec. 18, stated that North Korea is pursuing "nuclear, chemical and biological weapons that could threaten our homeland.”

The source in South Korea said Pyongyang is conducting heat and pressure resistance tests to see whether anthrax germs can survive at temperatures of 7,000 degrees or higher, the level an ICBM encounters when it re-enters Earth’s atmosphere.

There was an unconfirmed intelligence report that North Korea has already succeeded in such experiments.

Washington had apparently been aware for some time that North Korea is culturing anthrax.

That, according to a former U.S. government official, is the reason U.S. troops in South Korea have been vaccinated against smallpox and anthrax since around 2004.

In May 2011, the United States and South Korea carried out their first joint map exercise to deal with a terror attack using biological weapons.

The source in Seoul said Pyongyang is likely seeking to develop ICBMs loaded with anthrax because its nuclear-tipped ICBMs have yet to attain full-fledged capability to strike all of the U.S. mainland.