SEOUL , South Korea ( CNN ) -- South Korean and U.S. forces have been placed on a higher surveillance alert level , after North Korea threatened military action following its nuclear test earlier this week , the joint forces announced on Thursday . South Korean soldiers use binoculars to look at North Korea on Wednesday in Paju , South Korea . The `` Watchcon '' alert was raised to its second-highest level on Thursday , a government spokeswoman told CNN . The last time the joint forces raised the surveillance alert was after North Korea 's last nuclear test in 2006 , according to South Korea 's Yonhap News Agency . The separate five-stage combat alert level , known as `` Defcon , '' has not changed and remains at stage 4 , South Korean defense spokesman Won Tae-jae said at Thursday 's briefing , according to Yonhap . `` Additional intelligence assets , including personnel , will be deployed while reconnaissance operations over North Korea will increase , '' Won said , according to Yonhap . He declined to give specific details , the news agency said . Watch Hillary Clinton 's warning about 'consequences ' » . North Korea conducted a nuclear test Monday and fired five short-range missiles Monday and Tuesday . On Wednesday , the country threatened military action after South Korea joined a U.S.-led effort to limit the trafficking of weapons of mass destruction . There has also been recent activity at North Korea 's Yongbyon nuclear facility , according to U.S. officials , who cited information from U.S. spy satellites . The officials would not speculate about the type of activity . North Korea agreed in 2008 to scrap its nuclear weapons program -- which it said had produced enough plutonium for about seven atomic bombs -- in exchange for economic aid . But the deal foundered over verification and disclosure issues , and the North expelled international inspectors and announced plans to restart its main nuclear reactor at the Yongbyon complex . CNN 's Barbara Starr in Washington contributed to this report .
Joint command in South Korea elevate surveillance alert in response to N. Korean threat . Combat readiness remains unchanged , S. Korean military spokesman says . North Korea tested nuclear weapons , missiles , threatened military action .