A Senate subcommittee discovered yesterday that American military aid to Zaire - aid which the Carter administration has regularly described as "nonlethal" - may include M-60 tanks and armored personnel carriers.

Lucy Benson, assistant secretary of state for security assistance, confirmed that the administration has asked Congress for $30 million next year to begin payments on the offensive equipment for the embattled army of Zaire's President Mobutu Sese Seko.

In addition, the Defense Department has asked for $2.1 million in training funds, because there is apparently no one in the Zairian army who knows how to operate or maintain the 10 M-60 main battle tanks destined for delivery to Mobutu.

The proposal which came out under questioning at the tail end of a hearing before the Senate Foreign Assistance Subcommittee, Sen. Dick Clark (D-Iowa), the only subcommittee member who had stayed long enough to hear it.

Clark was disturbed on practical grounds - "I don't think they could use the tanks if they had them," he said - but also on policy issues.

"Zaire is a military dictatorship," Clark said. "They have openly violated some basic human rights. The government, to put it mildly , is rather corrupt.

"So why are we providing them with these arms?"

Benson answered that there is "no clear-cut answer" to that question. But she said Mobutu's disrespects for human rights is "not pervasive - it's an on-again, off-again thing."

The Central Intelligence Agency, in its 1977 National Baisc Intelligence Factbook, described the political system as follows: "Mouvement Populaire de la Revolution (MPR), only legal party, organized from above with actual grassroots popularity not clarely definable."

Air Force Gen. Howard Fish, director of the Pentagon's military assistance office, said in an interview after the Senate hearing that proposals for sending tanks to Zaire are "very tentative - just speculative."

But Senate sources said the administration had delivered to the Foreign Relations Committee an itemized list showing shipments of 10 battle tanks and 10 personnel carriers to Mobutu over a three-to-four year period.

Shipment of the tanks and personnel carriers to Mobutu was reportedly recommended by Army Gen. James Rockwell, who toured Zaire last year to evaculate its military needs.

Rockwell's report has not been made available to Congress, but some senators were familiar with its suggestions. Yesterday's hearing, however, offered the first public hint that American aid to Zaire would include offensive weapons.

Last week the State Department announced it was sending an immediate $13 million in "nonlethal" aide to Zaire, but said it had rejected Mobutu's pleas for arms.

Hodding Carter III, the State Department's press spokesman, said then that arms would not be sent to Zaire because "we see a need for an end to fighting, not an expansion of the fighting."

Clark, who visited Zaire six months ago, has cautioned against American military involvement in Mobutu's battle with Katangan insurrectionist troops based in neighboring Angola.

Some Senate aides familiar with Afican affairs speculated yesterday that Mobutu wanted the tanks mainly for prestige purposes - to impress his enemies with the scope of his support from the United States.

Earlier in yesterday's hearing, Benson and State Department counselor Matthew Nimetz danced delicately around the Carter administration's endorsement "in principle" of a military aid agreement with Turkey, that was negotiated by the Ford administration.

Senators repeatedly asked Nimetz whether the agreement was designed to pressure the Turks to soften their position in stalemated negotiations over control of Cyprus.

But Zimetz refused to state flatly that the administration has linked progress on Cyprus with full implementation of the proposed $1 billion Turkish aid agreement.