Six nonaligned leaders, seeking to prod the superpowers into banning all testing of nuclear weapons, have offered to monitor a comprehensive ban on underground tests with seismic devices on their own soil and reportedly are willing to implant devices near nuclear test sites in the Soviet Union and the United States.

Indian Prime Minister Rajiv Gandhi and Swedish Prime Minister Olof Palme, along with senior officials from Argentina, Greece, Mexico and Tanzania, approved the plan at a meeting in New York last Wednesday, according to sources familiar with their proposal.

Arms control advocates have argued that ending all nuclear tests eventually could halt the arms race, since the superpowers would be reluctant to deploy weapons that they had been unable to test.

A statement containing a broad outline of the proposal was presented Thursday by Indian Foreign Minister Baliram K. Bhagat to Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze, the sources said.

The plan is believed to have been among the topics discussed by Gandhi with Soviet leader Mikhail Gorbachev in a 90-minute meeting Saturday in Moscow.

The nonaligned leaders, in their latest appeal to the two nuclear weapons superpowers, proposed a 12-month suspension of all nuclear weapons tests.

In August, the Soviet Union announced a unilateral testing moratorium until Jan. 1.

The United States declined to join it, partly because of skepticism that such a prohibition could be verified. The United States has held several underground nuclear tests since Moscow's announcment.

The most unusual feature of the new proposal is its emphasis on verification, and especially the offer by the nonaligned states to assume a direct role in the monitoring process.

The lack of a precise means for detecting and measuring underground nuclear tests has been a stumbling block for the United States in previous test ban proposals, because of concern about possible Soviet noncompliance.

The nonaligned leaders' statement was approved by Gandhi and Palme, and by President Raul Alfonsin of Argentina, Prime Minister Andreas Papandreou of Greece, President Miguel de la Madrid of Mexico and President Julius Nyerere of Tanzania.

The proposal grew out of a "five-continent peace initiative" presented by leaders of the same countries in May, 1984, a summit meeting of the leaders in New Delhi this January and scientific studies of nuclear test monitoring undertaken by U.S. seismic experts.

The statement presented to the U.S. and Soviet governments by the six nonaligned leaders conceded that the problems of verifying a 12-month suspension of nuclear tests are "difficult, but not insurmountable."

The leaders said, "We are ready to offer our good offices in order to facilitate the establishment of effective verification arrangements."

The leaders also proposed establishing "verification mechanisms on our territories" to assist in monitoring the test ban.

Those familiar with the thinking of the leaders of the six nonaligned nations said that these "good offices" are meant to include the placement of seismometers, which are readily available devices for monitoring nuclear tests, operated by the nonaligned nations at nuclear test sites of the two superpowers, although it is unclear who would man the sites.

The chief technical adviser for the plan, Prof. Charles Archambeau of the University of Colorado, who is also a visiting scientist at the Defense Department's Center for Seismic Studies, said that nuclear tests could be detected and identified with such on-site seismometers down to "a very small fraction of a kiloton," even if there were attempts to muffle them in large caverns as U.S. officials have feared.

Archambeau said that the United States and Soviet Union each have only three or four sites that are used, or could readily be used, for nuclear testing.

Any attempt during the limited moratorium to create new test sites would have "a very high risk of discovery," Archambeau said.

State Department officials said last night that the message from the six nonaligned leaders is being studied within the administration.