South Korea and U.S. forces Thursday upgraded their alert level after North Korea announced that it is scrapping the armistice agreement, the South Korean Defense Ministry said.

The elevation of the five-stage "Watchcon" comes as tension runs high on the Korean Peninsula after North Korea conducted its second atomic test Monday and later scrapped the 1950-53 Korean War truce.

South Korean Defense Ministry spokesman Won Tae-jae said in a briefing that South Korea and the U.S. will deploy more intelligence assets and step up their reconnaissance operations over North Korea, according to Yonhap News Agency.

But Won said the five-stage combat alert level, "Defcon," remains at four. The elevation of the surveillance marks the fifth time South Korea and the U.S. have upgraded Watchon to the second highest level, he said.

The United States has 28,500 forces stationed in South Korea as a deterrent against North Korea.