Defense officials said the reported engine tests showed North Korea is continuing to pursue its missile programs but do not mean a launch is imminent. They said they have not detected any signs that North Korea's military has done anything suspicious.
The spy agency declined to comment.
The North Korean nuclear reactor was shuttered under a now-stalled international disarmament-for-aid deal. The reactor can be weaponized to make atomic bombs. Once it is operating, it can produce about 6 kilograms (about 13 pounds) of plutonium a year, enough for one or two bombs. Analysts estimate North Korea already has enough plutonium for between four to eight crude weapons.
North Korea threatened nuclear wars and issued a string of warlike threats this spring, but has gradually ratcheted down the threats and sought to resume cooperation projects with South Korea and stalled six-nation disarmament talks.
Earlier Tuesday, North Korea's military said its troops had been given an "emergency order" to re-examine operational plans and be ready to launch them to cope with planned trilateral maritime drills by South Korea, the U.S. and Japan this week. South Korea and U.S. officials said the drills in international seas off a southern South Korean island are routine training aimed only at improving readiness to respond to maritime disasters.
Chin Hee-gwan, a North Korea expert at South Korea's Inje University, said the North's latest threat shows its frustration at the lack of progress over its push to resume the lucrative cooperation projects with South Korea and the six-party talks. He said Tuesday's threat wasn't as serious as those it issued earlier this year and Pyongyang isn't expected to raise tension again anytime soon as it has to focus on reviving its economy.
The Korean Peninsula officially remains at state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 American troops are stationed in South Korea.