President Carter came under additional pressure yesterday to suspend the withdrawal of U.S. troops from South Korea as a Senate study group called for a halt.

"The new U.S. intelligence reassesment" of North Korean strength makes Carter's planned withdrawal of the 2nd Infantry Division too risky at this time, said their report.

Snes. Harry F. Byrd Jr. (Ind-Va.), Gary Hart (D.-Colo.) Sam Nunn (D-Ga.) and John G. Tower (R-Tex.) -- all members of the Armed Services Committee -- signed the report as members of the study group, while Sen. William S. Cohen (R-Maine) endorsed the findings at a news conference yesterday.

Nunn said at the news conference that he discussed the team's findings with Carter at the White House yesterday and found the president "very attentive" but nonocmmittal about calling off the planned withdrawals.

The Joint Chiefs of Staff are reviewing the military balance between North and South Korea with an eye to determining whether the withdrawals are still an acceptable risk. Carter is expected to withhold judgment until the new study is further along.

Defense Secretary Harold Brown went on record Saturday as believing that the higher estimates of North Korean strength would not by themselves derail the president's with-drawal plan. Said Brown:

However, some congressional insiders are predicting that Carter will suspend this year's planned with-drawal to avoid a fight with senators he hopes to win over to the new Strategic Arms Limitation Treaty with Russia, SALT II.

House Armed Services Committee members Samuel S. Stratton (D-N.Y.) and Robin Beard (R-Tenn.) are at the forefront of lawmakers there calling for a halt to the troop withdrawals.

The new intellingence estimate that has given withdrawal opponents additional leverage credits North Korea with an army of 550,000 to 600,000 trops rather than the previous figure of. 440,000, an increase of about 25 percent. The new estimate also credits North Korea with more armor and firepower.

"The reassessment casts grave doubt upon the validity of earlier judgments about the nature and stability of the Korean military balance that formed the basis of the administration's decision in May 1977 to withdraw U.S. ground troops from Korea," said the report of the Senate Armed Services Pacific Study Group.

At yesterday's news conference Hart stressed that the senators were not asking the president to scrap his withdrawal plan, only to suspend it until the risks were reassessed in light of the new intelligence information.

In June 1977, Carter announced that the ground force of about 33,000 troops would be withdrawn over a five-year period ending in 1982. The 1978 withdrawal, which was supposed to total 6,000 troops, was slowed to about 3,500. The other 2,500 are scheduled to be withdrawn this year, with the big exodus coming later.

Nunn said going ahead with the planned withdrawals in the face of the new intelligence estimates would "reduce deterrence" and thus "increase the possibility" of war on the Korean peninsula.

Hart, in individual views appended to the report, said "complete withdrawal of U.S. ground forces from Korea should remain our goal."