In a statement sure to outrage U.S. shippers, Maritime Administrator Warren Leback Thursday said he will work to secure cargo reservation measures in any new maritime agreement with the Soviet Union.

One-third of the grain and other bulk commodities moving between the United States and the Soviet Union should be reserved for American-flag shipowners carrying such goods, as long as they have "competitive rates" and ships available to carry the cargo, Mr. Leback said in a speech to the American Association of Port Authorities here.Under such an agreement, another one-third of the traffic would be reserved for Soviet-flag ships and the final third would be open to the vessels of other nations.

Such a provision could have an impact on the export of American grain to the Soviet Union because U.S.-flag rates generally are higher than world rates.

Steven McCoy, president of the North American Export Grain Association, has already loudly objected to cargo reservation provisions in any new agreement. Other shipper groups have objected as well.

Mr. Leback noted that the idea of reserving cargo for U.S. carriers tends to "raise the hackles" of American shippers and suppliers. But, he added, if ''the cargo originates here, it's only fair that American-flag carriers" be entitled to carry some of it, if they are "competitive."

The United States and the Soviet Union are in the midst of negotiating a bilateral maritime agreement. The third round of talks is scheduled for April 26 in Moscow.

Mr. Leback deviated from his prepared text because the U.S. position in maritime talks has been misrepresented in the press, he said. Mr. Leback has declined in the past to be interviewed about the progress of the talks.

U.S. shippers, particularly bulk agricultural suppliers, oppose cargo reservation for U.S. carriers as unnecessary protectionism that will reduce the competitiveness of U.S. exports. The costs of using U.S.-flag carriers are typically higher than those of using a foreign-flag carrier.

"If American-flag carriers are offering a competitive rate and have ships available then, yes, I think they should get a share and not be discriminated against," Mr. Leback said in a brief interview after his speech.

Mr. Leback also said the United States would get protection for American carriers in any agreement with the Soviet Union involving access to trading lanes between the United States and third countries.

The Soviet Union is pushing for such so-called cross-trade access. U.S.-flag carriers are worried Soviet carriers could undercut their rates in those trade lanes.

"American carrier company investments need to be protected," Mr. Leback said. U.S negotiators would make sure safeguards are in place, he added.