"Reinforcing war deterrent is one hundred times justifiable," said Rodong Sinmun, newspaper of the ruling communist party, in a commentary attacking the war games.
The phrase normally refers to its nuclear weapons programme.
The North has made such threats before, but the latest commentary comes amid stalemate in an international nuclear disarmament deal and uncertainty about the health of leader Kim Jong-Il.
Rodong criticised the annual exercise dubbed Ulchi Freedom Guardian, which was held in South Korea from August 18-22.
"They were rehearsing... to launch a preemptive strike on North Korea," it said, accusing the US of launching various other joint military exercises with South Korea and supplying it with modern weaponry.
"We will strengthen the war deterrent as long as the US warlike forces continue the policy of attempting to stifle the DPRK (North Korea)," it said.
Officials in Seoul and Washington say the joint exercise is defensive.
The North tested an atomic bomb in October 2006 but later reached an aid-for-disarmament deal. This is deadlocked by a dispute over how to verify the North's declared atomic operations.
South Korean officials say Kim, 66, is recovering after undergoing brain surgery following a stroke last month. Other reports say his illness may have impaired his decision-making abilities.
Seoul's defence ministry spokesman said Tuesday that US and South Korean troops would launch a joint landing exercise in November.
The spokesman told a briefing: "South Korea has staged a 'Hoguk' landing drill either for itself or jointly with the United States annually. The 'Hoguk' drill is due in early November this time."
"Consultations are still underway on how many US troops will take part or how large in size the entire force will be," he said without elaborating.
Yonhap news agency said Monday that General Walter Sharp, who commands 28,500 US troops here, has proposed an exercise involving more than 10,000 US and South Korean marines.
But Seoul is lukewarm for fear that any large-scale drill could further chill relations with the reclusive regime in Pyongyang, according to Yonhap.