Defense officials confirmed yesterday that Soviet arms control negotiators have agreed to an American proposal to allow cruise missiles of 1,550-mile range to be tested over the next three years on ships and land bases as well as on bombers.

This is what Defense Secretary Harold Brown meant, these officials said, when he told a news conference in Brussels on Wednesday about the "substantially longer" range to be allowed on the cruise missile under the arms pact now being negotiated.

Said Brown: "The range limitation for sea and ground launch cruise missiles insofar as development and testing is concerned will not be limited to 600 kilometers [about 360 miles] as the agreement is shaping up . . . It will be substantially longer than that."

The Soviet willingness to agree to allow long-range cruise missile to be tested on ships and land bases as well as bombers is being described by administration officials as a major concession that brightends prospects for an arms control agreement between the United States and the Soviet Union.

Administration officials two weeks ago described the Soviet shiift on the cruise missile as a helpful concession. But Brown's statement on Wednesday was the closest thing to an official, public statement on he matter.