The development follows U.S. President Donald Trump's order to scale back the exercise.
The JCS said the allies agreed to adjust the UFS schedule to run from Aug. 17 through Aug. 21. The exercise had originally been scheduled to continue through Aug. 27.
The decision came after Trump said last Sunday that he had instructed the Pentagon to "substantially" reduce the exercise with South Korea.
The allies will also scale back some of combined field-training drills as part of the UFS exercise, while Seoul and Washington continue to discuss further details.
South Korea and the U.S. had initially planned about 14 such drills at the battalion level or above during this year's UFS.
The annual exercise, which began Monday with about 18,000 South Korean troops participating, is designed to strengthen the allies' combined defense posture and rehearse responses to potential security contingencies on the Korean Peninsula.
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