A new, more austere U.S. defense strategy unveiled Thursday gives up on fighting major wars overseas and reduces active-duty troops from 570,000 to 470,000. The aim is to cut more than US$450 billion in defense spending over the next decade.

The new strategy would make it virtually impossible for the U.S. military to fulfill a pledge to South Korea to deploy 690,000 troops on the Korean Peninsula in an emergency.

"Washington recently told us that the revision of its defense strategy will have no significant effect on South Korea," a government source said. "But there'll inevitably be a sea change in operational plans and troop augmentation in case of an emergency here."

South Korea would then have to take more responsibility for itself and shoulder a more substantial burden for its own defense, observers said. Washington is to hand over full operational control of South Korean troops to Seoul in December 2015.

Under a current operational plan for a full-scale war on the peninsula, the U.S. pledges to send about 690,000 reinforcement troops to the peninsula 90 days after a war breaks out. But under the troop reduct ion plan that much personnel will no longer be available.

The number of reinforcements could be cut to less than 200,000, some experts speculated. "It's highly likely that military strategies including [the operational plan] will be revised in a way that will cut the number of reinforcement troops," said Song Dae-sung, head of the Sejong Institute.