President Reagan has decided to ask the Soviet Union for changes in two treaties limiting some nuclear weapons tests that were negotiated and signed by the Nixon and Ford administrations during the mid-1970s but were never submitted to the Senate for ratification, senior administration officials said yesterday.

The president's action at a National Security Council session Monday effectively puts off for now any resumption in negotiations for a total ban on nuclear testing. Those negotiations have been suspended since November, 1980.

A senior administration official said Reagan wanted to strengthen protections against violation of the two treaties that have not yet been ratified by the Senate.

One is the Threshold Test Ban Treaty, which the United States and the Soviet Union signed in 1974 to prohibit underground nuclear tests greater than 150 kilotons.

The other is the Peaceful Nuclear Explosions treaty, signed in 1976 to forbid underground nuclear explosions over 150 kilotons for peaceful purposes such as mining.

Efforts have been under way for more than two decades to bring a halt to all nuclear weapons testing. The first benchmark was the 1963 U.S.-Soviet treaty, which outlawed nuclear tests in the air, sea and outer space, and which remains in effect.

The next steps were the two treaties covering underground tests, negotiated during the Nixon-Ford years. These are the ones Reagan now seeks to modify. The final but elusive goal, embraced by every administration since John F. Kennedy, has been a total ban on nuclear testing.

In detailing Reagan's decision yesterday, the senior official said the administration intends to ask the Soviets for "improved verification procedures" in the treaty protocols that would give each side better information on compliance with the agreements.

The current verification methods are "rudimentary" and need to be strengthened, the official said, but "we do not know if the Soviet Union will want to do this."

Reacting to the president's decision, Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) said that while the treaties "must be adequately verifiable," any new negotiations with Moscow should not "become protracted and result in a collapse of the two treaties."

A more critical Senate Minority Whip Alan Cranston (D-Calif.) said the president's "extreme position bodes ill for for the success of any arms control negotiations" between Reagan and the Soviets.

The senior administration official said the United States intends to continue complying with the two treaties while seeking to reopen talks with the Soviets aimed at modifying or amending them.

Both nations agreed to honor provisions of the treaties in the 1970s, but "seismic signals from the Soviet Union have been of a sufficient magnitude to call into question Soviet compliance" with the 150-kiloton limit, said the official, who spoke to reporters at the White House on the condition that he not be identified.

"The Soviets have always asserted, when challenged on this point, that they have not violated the agreement. But this underscores the problem," the official said. "Our security, in the president's judgment, requires that we do not agree to an unverifiable treaty."

The official said the United States had lodged protests with the Soviets over underground nuclear tests that appeared to exceed the limit. According to another administration official, there have been 11 such tests, but the seismic data collected by the United States has not been conclusive enough to prove "beyond a reasonable doubt" that the limit of 150 kilotons was exceeded. A weapon of 150 kilotons is more than 11 times the size of that dropped on Hiroshima.

The 1963 agreement between the United States and the Soviet Union banning all nuclear testing above ground, in water and outer space eventually was joined by 120 other nations. But a comprehensive test ban treaty that would prohibit all nuclear tests effectively has been caught up in disagreements between Washington and Moscow over whether compliance can be verified to the satisfaction of each side.

The president's decision Monday in effect rules out any resumption of those negotiations while changes in the two other treaties on underground testing are being worked out, the senior administration official said.

"We think that the first thing that has to be done is to get to what we have, and let's make that workable and effective and that is to strengthen the verification provisions of the TTBT Threshold Test Ban Treaty and the PNE Peaceful Nuclear Explosions treaty ," he said.

The official added, however, that the United States continues to participate in a panel studying the problem of verification in a test ban treaty that is part of the U.N. Committee on Disarmament in Geneva.

"We think that a comprehensive ban on nuclear testing must remain a component of this administration's long-term arms control objectives, but there are problems to be overcome, and these deal essentially with verification," the senior administration official said. "My experience has been that the Soviets simply have not or have refused to accept measures that would assure effective verification."

One administration official said yesterday that modifying the two nuclear testing treaties was not a high priority of the administration. Instead, the "emphasis" in arms control would continue to be limitations on the number of strategic nuclear weapons deployed and stockpiled by the two superpowers, he said.

Sen. Edward M. Kennedy (D-Mass.) said yesterday that he plans to introduce a resolution calling for early resumption of the comprehensive test ban treaty negotiations and urging the administration to send the two other treaties to the Senate for ratification.

"This decision flies in the face of a nationwide call for an immediate freeze on the testing, production and deployment of nuclear weapons," Kennedy said.