President Reagan and Soviet leader Mikhail Gorbachev reached an accord on space-based missile defenses yesterday that raises as many questions as it resolves, according to senior U.S. officials who participated in the discussions.

As the end of their three-day meeting drew near, the two leaders accepted a complicated formula on the troublesome missile-defense issue that skirts a vigorous and lasting dispute over scientific research and the direction of the U.S. and Soviet weapons policies.

"I would not use the word breakthrough . . . because there is still lots of work to be done on this issue," said a senior U.S. arms control official last night.

"What we have erected is a much better platform to do the work . . . {that} takes the edge off" persistent U.S. and Soviet wrangling over Reagan's Strategic Defense Initiative (SDI) program to develop and deploy a ballistic missile defense, the senior official said.

Perhaps reflecting lingering uncertainty about whether the issue has been resolved or simply finessed for now, neither Reagan in his televised speech nor Gorbachev in his press conference played up the accord on SDI as a major achievement.

According to the compromise language developed as an "instruction" to U.S. and Soviet arms negotiators in Geneva, both sides agreed to observe the 1972 Antiballistic Missile (ABM) Treaty "as signed," and may not withdraw from the treaty or abrogate its provisions for a period of time to be agreed upon later.

The period of time is expected to be somewhere between the current Soviet position of 10 years and the current U.S. position of seven years.

The accord was a compromise between the initial Soviet position that both sides should observe the treaty as it was "signed and ratified" in 1972 and the U.S. position that both sides simply agree to observe the treaty without further qualification.

The Reagan administration objected to any reference to the treaty's ratification because the ratification record has been interpreted by Congress as supporting a view that realistic space tests are barred under the treaty. The administration has embraced an interpretation of the treaty allowing such tests, which it believes are necessary to pursue the SDI research program. But Congress has voted to prohibit the administration from operating under that interpretation through next Oct. 1.

The accord does not mean the two sides are agreed on whether missile defenses are valuable or on the type of scientific research that can be pursued during the period in which both sides abide by the ABM Treaty, U.S. officials said.

"The Soviets have a different view of strategic defense, and they strongly made the case that . . . we should not pursue" SDI, a senior administration official who advises the president on national security issues said at a press conference yesterday. "The president just as strongly reiterated his case that we believe . . . {SDI} is the direction we should be moving in the future."

In his press conference yesterday, Gorbachev said the Soviet Union is "resolutely opposed to SDI. We shall not build up an SDI in our own country . . . . If the Americans have all that much money, let them squander that money away."

The two sides agreed to paper over their dispute by explicitly allowing missile defense "research, development and testing as required, which are permitted by the ABM Treaty."

The senior official briefing reporters yesterday said the administration has interpreted this convoluted statement in the two leaders' joint communique to mean that realistic space tests can be conducted as soon as the congressional legislation constraining them expires next October. "We have not given up anything on SDI with respect to our ability to pursue the program," the official said.

Other U.S. officials acknowledged, however, that the Soviets still maintain the ABM Treaty bars realistic space tests of missile defense weapons. By agreeing to such ambiguous phrasing, the officials said, the Soviets have essentially reserved the right to complain about a future U.S. decision to conduct such tests.

"It doesn't nail that down," the senior arms control official said.

"On the basis of what is in the {joint} statement, I would see no basis for them to object" to U.S. space tests, the presidential adviser said. But he added, "I don't know what they might or might not do in the future" in response to U.S. tests.

The statement in the joint communique specifies that, three years before the end of the period in which both sides have pledged not to withdraw from the treaty, they must conduct "intensive discussions of strategic stability." This obligation, suggested by the Soviets, will provide each side an opportunity to tell the other why SDI does or does not contribute to "stability."

Afterwards, each side is "free to decide its own course of action," according to the joint communique.

This phrase was interpreted by some officials as a substantial concession by the Soviets because it leaves the United States free to carry its SDI research and development to fruition.

"If an effective strategic defense capability has been developed, we will have the ability and the right to deploy it," the presidential adviser said. "Nothing in today's joint statement . . . constrains that."

But other officials called the language troubling because it allows the Soviets to respond as it sees fit to deployment of a U.S. missile defense system.

The Soviets have maintained in Geneva negotiations that any U.S.-Soviet treaty capping the number of strategic offensive weapons would "cease to be in force if either party proceeded with practical development and deployment of an ABM system beyond the provisions of the ABM Treaty."

Several U.S. officials said last night that nothing Gorbachev said or agreed to during the summit indicated he has abandoned this position, nor did the United States indicate it was willing to accept this position.