The Joint Chiefs of Staff have formally recommended that President Carter suspend his planned withdrawals of American combat troops from South Korea.

The Chiefs, sources said, told the president in their new report on the North Korean military threat that both the increased size of the North Korean military and its recent muscle-flexing make withdrawals too risky at this time.

The Chiefs' report is based on their reaction to new intelligence data on North Korea put together by the Central Intelligence Agency and the Defense Intelligence Agency, U.S. intelligence officers have publicly admitted that they underestimated the size of the North Korean military. For example, North Korean troop strength is now put at between 550,000 and 600,000 instead of the previous estimate of 450,000

President Carter and Defense Secretary Harold Brown are scheduled to meet in Korea later this week to observe conditions on the spot and to discuss future U.S. military aid for South Korea.

In the discussions that preceded the president's trip to South Korea, high-level officials at both the Pentagon and State Department reportedly agreed that South Korea should be told that the United States intends to sell them modern F16 fighters, but the delivery dates would not be stated. This would amount to the administration's earlier approval "in principle" of South Korea getting the hot fighter-bomber.

South Korea wants the F16, starting with the delivery of about 40 of them, but the administration has been reluctant to risk escalating the arms race between South and North Korea by supplying such a modern weapon.

Brown has said that he does not expect the president to announce his plans for Korea until after his return to Washington.

Carter, during the 1976 campaign, promised to withdraw American troops from South Korea. He suspended the withdrawal, however, last February after about 3,400 of the 27,000 U.S. troops planned for withdrawal had been sent home.

Sources said that Carter's timetable before receiving the Chief's latest report called for withdrawing 9,000 troops in 1980.

The Chiefs' latest report to the president calls for a suspension of the withdrawal, not abandoning the idea altogether.