The delay was announced three days after North Korea urged the US to cancel its war games with the South, scheduled for August 20-31, warning that the exercises would have a "catastrophic impact" on ongoing six-nation talks on the North's nuclear disarmament.
North Korea routinely criticises the annual South Korea-US "Ulchi Focus Lens" drill as a rehearsal for an invasion of the North.
American and South Korean officials have repeatedly said the exercises, held since 1975, are purely defensive.
The joint military drill is largely a computer-simulated war game, involving about 10,000 US troops in South Korea and abroad, according to the US military.
About 28,000 US troops are stationed in South Korea as a legacy of the 1950-53 Korean War, which ended in a ceasefire, not a peace treaty, leaving the two Koreas technically at war.