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.
What did North Korea test? What is the status of the combat alert level? What was unchanged?
nuclear stage 4, "Defcon,"
question: What did North Korea test?, answer: nuclear | question: What is the status of the combat alert level?, answer: stage 4, | question: What was unchanged?, answer: "Defcon,"