A senior Soviet official charged here yesterday that the United States has reneged on an understanding about testing and development of space-based defensive weapons reached at last month's Washington summit, creating a serious obstacle to agreement on a treaty reducing long-range offensive arms.

Georgi M. Kornienko, a senior Kremlin official, said the U.S. shift in the Geneva negotiations showed "disrespect for the joint statement" issued by President Reagan and Soviet leader Mikhail Gorbachev at the end of their Dec. 8-10 meeting.

The joint statement used ambiguous language to bridge longstanding U.S.-Soviet differences over development of space-based antiballistic missile (ABM) systems like those envisioned in Reagan's Strategic Defense Initiative, or "Star Wars" program. Kornienko said at a Soviet Embassy news conference that instead of leaving this "conceptual dispute" to be resolved at "some later time" as agreed at the summit, "the U.S. delegation {to the Geneva arms talks} again tries to convince the Soviet delegation that it would be good to move forward to deployment of outer-space ABM systems."

Kornienko is senior deputy to Anatoliy F. Dobrynin, the former Soviet ambassador in Washington who now holds the powerful position of secretary of the Communist Party Central Committee for foreign affairs.

A senior U.S. official said Kornienko, in his Washington conversations with administration arms-control experts, had also objected strenuously to the title of the U.S. draft treaty on strategic defense which was presented Jan. 22 in the confidential Geneva talks.

The title is: "Treaty Between the United States of America and the Union of Soviet Socialist Republics on Certain Measures to Facilitate the Cooperative Transition to the Deployment of Future Strategic Ballistic Missile Defenses."

This title is offensive to the Soviets because they do not accept the idea that space-based ABM defenses should be deployed and they repeatedly have rejected any such "transition." Deployment of space-based missile defenses is barred by the 1972 Antiballistic Missile Treaty.

Kornienko was told that the title of the U.S. draft is not important, according to this official. Another U.S. source said that the title placed on the U.S. draft was "really rubbing their noses in it."

He said the purpose of this language was to stake a claim to a permissive interpretation of the ABM Treaty allowing realistic "Star Wars" tests in space, which the Soviets have rejected.

Kornienko said after the news conference that it would be impossible to resolve their dispute over the meaning of the ABM Treaty in time for the Moscow summit between Reagan and Gorbachev expected in late May or early June. He said the Soviet side would not in any circumstances accept the U.S. "broad," or permissive, interpretation of the treaty.

A U.S. official, asked about Kornienko's remarks, said Soviet negotiators at Geneva have proposed treaty language that implies a restrictive interpretation of the ABM Treaty in recent weeks, amounting to a similar shift away from the summit accord.

These developments suggest that either the United States or the Soviet Union will have to abandon its view of the ABM Treaty or the two sides will have to return to a vague formulation that obscures their differences, if a new strategic arms treaty is to be concluded at the Moscow summit meeting, as Reagan and Gorbachev have sought.

Kornienko was in Washington at the invitation of Congress as part of a delegation from the Supreme Soviet, Moscow's legislature, which is charged with formal ratification of the Intermediate-Range Nuclear Forces (INF) Treaty signed at the Washington summit.

Delegation Chairman A.E. Voss said "expanded hearings" will be held by the foreign relations committees of the bicameral Supreme Soviet on the INF pact in the next several months.

"We regard it as our task to convince the doubters -- and there are some doubting people -- and to respond to the questions of the Soviet people," Voss said.

Meanwhile, the U.S. Joint Chiefs of Staff have concluded in a report to Congress that elimination of all U.S. and Soviet ballistic missiles, first suggested by Reagan at a 1986 summit meeting, could be costly and dangerous to U.S. security.

The report said the proposal, which Gorbachev rejected, "should not be contemplated" again without assurance that it could be adequately verified.

Sen. Sam Nunn (D-Ga.), chairman of the Senate Armed Services Committee, said the report demonstrates Reagan's proposal "had not been thought through."