The United States and the Soviet Union failed to resolve their differences on a new strategic arms limitation treaty (SALT) in lengthy negotiations here yesterday, but both sides pointed expectantly to another meeting scheduled at the White House Saturday.

It was not clear whether Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei A. Gromyko were able to nail down any additional agreements in their four-hour session at the United States mission to the United Nations as negotiations entered an unusually intensive phase.

President Carter, speaking at news conference two hours after the talks ended, said he did not see any "insurmountable obstacle." Carter said he plans "to capitalize upon the progress that I hope that Vance and Gromyko are making now" when he meets with the two of them at the White House Saturday morning. He said Vance and Gromyko are negotiating "aggressively and in good faith."

Gromyko on Wednesday presented a set of Soviet counterproposals, which some sources described as relatively forthcoming, on the outstanding issues. Vance gave U.S. responses in yesterday's meeting. Then in the diplomatic give and take, Gromyko offered a futher Russian reaction.

Carter may present Gromyko with a "bottom line" set of U.S. suggestions to settle the remaining problems in the White House meeting Saturday, according to an official source.

As described by the veteran Soviet diplomat in a brief talk with reporters following yesterday's meeting, negotiation of the few remaining issues is a tradeoff process of interwined compromises and commitments.

"If I had a ball of twine at my disposal I could show you that graphically," Gromyko said.

Vance spoke to reporters of a less intricate process, saying it is necessary to proceed "brick by brick" toward final agreement.

Asked if the current talks have brought the superpowers closer to a new treaty, Vance replied, "Let's wait and see what happens after the (White House) meeting on Saturday."

The U.S. side has placed emphasis on the need for Soviet concessions in this late stage of SALT negotiations, speaking privately of pressure of hold the line on several still-disputed provisions to satisfy key senators whose votes will be needed in the ratification proces.

Carter seemed to place the burden on the Russians at this stage by telling his press conference. "If the Soviets are forthcoming and cooperative and are willing to compromise some of their positions, we will have an agreement."

The president spoke without enthusiasm in answer to a question, of proposals that mobile U.S. missiles travel secretly between alternate launching points to confuse Soviet targeting in the mid-1980s. This plan has been suggested as a means of diminishing the vulnerability of U.S. land-based missiles as Soviet nuclear power and accuracy improve. Carter said the proposal has "some very serious defects" including the difficult problem of verification and described it was one among many ideas" under study.

He said he anticipated a decision would be made by the end of the year on programs to deal with future vulnerability of U.S. land-based missiles and that he expected to explain future U.S. strategic programs, probably for the next five years, in addressing the American people after the conclusion of a SALT agreement.

While most of the Vance-Gromyko meeting centered on SALT, the two men also discussed the comprehensive nuclear test ban treaty, which also is close to a final agreement, and U.S. Soviet negotiations for control of conventional arms sales aboard. The conventional arms talks, which are to resume later this year, are considered vital to continuation of the Carter administration's program of unilateral restrictions on U.S. weapons sales.

The two foreign ministers also discussed the Camp David agreements between Egypt and Israel. The Soviet Union has sharply criticized the agreements, saying they tend toward a separate peace between those two nations without resolving the Palestinian problem. According to American officials, the Soviets are bitterly critical that they have no role in the current middle East negotiations.