A House Armed Services subcommittee yesterday began "a frontal assault" on President Carter's Korea policy by calling Maj. Gen. John K. Singlaub to testify on why he believes withdrawing Americans troops from South Korea would lead to war.

Singlaub, whom Carter fired Saturday as chief of staff of the American military command in Korea, testified that he knew of no senior American or South Korean officer who favored the President's withdrawal plan, and said the Joint Chiefs of Staff have yet to provide the Korean command with a rationale for it.

Despite complaints from members that the Armed Services Investigations Subcommittee was conducting a "frontal assault" on Carter's withdrawal plan through "wholly inappropriate" hearings, subcommittee Chairman Samuel S. Stratton (D-N.Y.) said after yesterday's public session that the Joint Chiefs of Staff will be called next to discuss Korea.

While House opponents of withdrawing American ground troops from Korea use military officers to make their case to the public, a growing number of Senate leaders are beginning to express their own personal doubts about Carter's plan, portending a fresh foreign policy fight for the President.

Sen. John G. Tower, (R-Tex.) said members of the Republican Policy Committee, which he heads, expressed their uneasiness at Tuesday's meeting over the planned troop withdrawals. Tower said he opposes the Carter plan.

Chairman John C. Stennis (D-Miss.) of the Senate Armed Services Committee said yesterday that "I would expect there will be full discussion in the Congress" about the withdrawal.

Singlaub said yesterday that as far as he knows, Carter has decided only on the first two increments of the withdrawal of the 33,500 Army ground troops. Carter plans to have all those troops out of Korea by 1982.

Singlaub, under questioning by subcommittee members yesterday, said Carter "did not give me an explanation" of his troop-withdrawal plan at their WHite House meeting Saturday.

Carter summoned Singlaub to the White House to discipline him for stating in an interview published in The Washington Post on May 19 that "if we withdraw our ground forces on the schedule suggested, it will lead to war." Carter ordered the Army to assign the two-star general to a new post, which has not yet been announced.

Singlaub did not back off from the controversial quote yesterday but said that his remarks were in the context that South Korean commanders believed American withdrawal would lead to war and that he agreed with them.

Defense Secretary Harold Brown said at the National Press Club yesterday that Singlaub had been informed by Gen. Bernard W. Rogers, Army chief of staff, about Carter's withdrawal decision before he warned against it. "Because he knew and expressed an opinion different from that policy," Brown said, it "became impossible" to keep him in the chief-of-staff job in Korea.

Singlaub said that long before the newspaper interview he and other officers at the American command in Seoul had expressed through military channels their reservations about various troop-withdrawal plans being weighed in Washington.

"Some, would lead to disaster," Singlaub said, but on the strength of comments sent from the American command in Korea to Washington "we ended up with the least undesirable" of the withdrawal options submitted in February, March "and perhaps into April."

"We were never asked to comment on the desirability of the withdrawal," Singlaub said of the Seoul command, where he was the third-ranking Army officer and fourth in the chain of command.

"We are being asked the wrong questions" in assessing the various withdrawal options, he said. "My hope" was that the command's reservations about the option would trigger "a re-look" at the whole idea of withdrawing American ground troops at this time.

Singlaub said he believed it was in February when his command sent a request through channels to the Joint Chiefs of Staff for an explanation of the withdrawal policy - an explanation that has not been received.

Rep. Lucien N. Nedzi (D-Mich.) retorted that President Carter was under no obligation to explain his decision to military officers. "This could be helpful but not necessary," Nedzi said in recalling the Army as a place where orders are carried out unquestioningly. "Should the President go to Korea and hold a mass meeting with the division?? Nedzi asked Singlaub.