Soviet Ambassador Anatolly F. Dobrynin said yesterday that he expects a new American-Soviet agreement on limiting strategic weapons by the end of this year.

But U.S. Defense Secretary Haroid Brown, apparently sensitive to mounting criticism that the Untied States is conceding too much in negotiations now underway with the Soviets in Geneva, took a more cautions view.

"It takes two sides to make an agreemetn," Brown said yesterday on "Issues and Answers" (ABC, WJLA). "I wouldn't say there are new stumbling" blocks to anew accord, he said, "I would say there are old stubling blocks not yet surmounted. It will take some time to work those out . . ."

Dobrynin's remarks in an interview for NBC's "Today" show - to be aired this morning - marked the first time a senior Soviet official had publicly predicted a time for concluding the strategic arms limitation talks (SALT).

The Soviet offical also said he expects a summit meeting between President Carter and Soviet leader Leonid Brezhnev once a new SALT accord is reached.

"We are rather close" to a SALT II agreement, Dobrynin said, "How long it will take, I think nobody could tell you eith precision.

"My guess will be . . . by the end of this year. This is my own (judgment) . . . But I think this will be. As things stand now, they're not as bad as sometimes presented by some (congressional) committees in your country." Dobrynin said, referring to criticism of the Carter administration's efforts to reach a SALT accord.

Much of that griticism - chiefly from Sen. Henry M. Jackson (D-Wash.) and former Deputy Secretary of Defense Paul H. Nitze - contends that the admonistration is seriously weakening this country's nuclear defense posture in the negotiations.

For example, Nitze - who was a SALT negotiator from 1969 to 1974 - said last Tuesday that by 1985 current U. S. policy would give the Soviets near-equality in heavy bombers and a 10-to-1 advantage in land-based intercontimental ballistic missiles.

Nitze also said that with the Soviet advantage in the ability to propel nuclear payload, combined with improved accuracy, their ICBNs could destroy around 90 per cent of U. S. Minuteman silos in a nuclear attack.

Brown publicly conceded yesterday what Pentagon officials have been saying on background in the past - that U. S. Minuteman ICBMs would be vulnerable to a Soviet nuclear first strike sometime before 1985.

But, he added: "At the same time. I think that the Soviets would not be, or at least should not be, confident by that time that the Minuteman would not survive. In other words survival of the Minuteman would be in doubt, but survival of the Minuteman against a surprise attack is not the same thing as survival of the United States.

"Our deterrence consists of a number of different systems of which land-based missiles are but only one."

He said the United State could respend to a surprise Soviet attack with manned bombers and submarinelaunched missiles "in a circumstance where they could debastatingly retaliate against the Soviet Union."

He said the product of such a reaction would be "a war that both sides would have lost."

Brown said he believes that United States and Soviet Union have a "raugh party" in nucleaar capability and that criticism that the Carter administration is giving up too much in the SALT talks "is not a correct estimate" of this country's bargaining position "now or in the future."

"Let me try to put it into perspection. I believe that most of the opponents of the way the strategic arms limitation negotiations have gone would say thatwe are roughly in parity with the Soviets in terms of strategic capabilities, now, at this time," he said.

"Many of them [crities] are very xoncerned about the future situation, and some of them believe that a SALT agreement would assure the Soviets numerical superiority . . ." he said.

"I don't believe that is the case, myself, I believe we shouldn't yet draw final conclusions about an agreement because there remain a great many details, important details, that have to be settled. Until they are settled, I would not want to make an overall military evaluation."