http://www.bbc.co.uk/news/world-asia-pacific-10736538
The comments came as Asian foreign ministers met in Vietnam for a regional security forum.
The forum has been dominated by the crisis resulting from North Korea's alleged sinking of a South Korean warship in March.
The US has accused Pyongyang of "provocative" behaviour and on Wednesday announced new sanctions.
North Korea's delegation spokesman at the Association of South East Asian Nations (Asean) Regional Forum (ARF) in Hanoi said the military exercises - which begin on Sunday - were an example of 19th century "gunboat diplomacy".
"It is a new century and Asian countries are in need of peace and development, and DPRK [North Korea] is also moving to that end," said Ri Tong-il.
He said the exercises went beyond defensive training and would involve "sophisticated weapon equipment".
"It is a threat to the Korean peninsula and the region of Asia as a whole. And the DPRK's position is clear: there will be a physical response to the threat imposed by the United States militarily."
Washington and Seoul say the war games - involving the aircraft carrier USS George Washington, 20 other ships and submarines, 100 aircraft and 8,000 personnel - are intended to deter North Korean aggression.
China has criticised the plans and warned against any action which might "exacerbate regional tensions".
But Japan is sending four military observers, in an apparent endorsement of the drills.
US Secretary of State Hillary Clinton, who is attending the forum, declined to comment directly on North Korea's comments, but said the door remained opened for it to return to talks if it committed to irreversible de-nuclearisation.
In comments earlier in the day, she accused Pyongyang of launching a "campaign of provocative, dangerous behaviour".
John Sudworth BBC News, Seoul
It is unclear, of course, how carefully the North Korean spokesman weighed his words before unleashing his latest verbal broadside. But the talk of a "physical response", if taken at face value, seems troubling.
Angry rhetoric from North Korea is nothing new - it serves both a domestic political purpose, keeping the enemy constantly in the forefront of its citizens' minds, and an external one, by raising tension when Pyongyang believes it is in its interests to do so.
At times the language turns flamboyant, even poetic. So it is difficult to know what to make of the latest, threatened "physical response", but on past form, it probably should not be taken at face value.
On Wednesday, the US announced it was to impose new sanctions on North Korea, aimed at halting nuclear proliferation and the import of luxury goods.
The Asean Regional Forum was attended by foreign ministers from 27 nations, including China, Japan, North Korea and the US.
The BBC's Rachel Harvey in Hanoi said there had been hopes that the forum could be a chance to reduce tensions on the Korean peninsula.
But if anything, the proximity of the protagonists seemed to inflame sentiment, our correspondent adds.
South Korea sought to rally support behind a statement condemning North Korea over the warship sinking.
But a closing statement from the forum only expressed "deep concern" over the incident, echoing a UN statement that fell short of blaming Pyongyang.
China - which is North Korea's closest ally - has been reluctant to take strong measures against its unpredictable neighbour.
The Cheonan warship sank in March near the disputed inter-Korean maritime border with the loss of 46 South Korean sailors.
An international investigation said it was sunk by a North Korean torpedo, but Pyongyang has rejected the charge and demanded its own inquiry.
In a separate development on Friday, North Korean and US-led UN Command military officials held rare talks on the sinking in the truce village of Panmunjom in the Demilitarised Zone (DMZ) between the two Koreas.
The talks lasted two hours, Yonhap news agency said. The results were not known, but they were believed to have set out the details for further higher-level talks on the issue.