LONDON, DEC. 16 -- The Soviet Union is "entirely premature" in demanding a United Nations-flagged naval force for the Persian Gulf, and the first step should be agreement on an arms embargo against Iran, Secretary of State George P. Shultz said here today.

Shultz told reporters that the Soviets, after months of rejecting U.S. calls for agreement on a U.N. Security Council resolution on an arms ban, have "expressed a readiness" to support such a resolution.

Iraq has accepted a Security Council resolution calling for a cease-fire in its seven-year-old war with Iran. But Iran has not, and now the Security Council is considering a draft resolution calling for an arms embargo to punish it.

Shultz said the Soviets "have said they want to go forward with a parallel effort at enforcement of the resolution, and they seem to have in mind naval enforcement." He said a peace-keeping force would be appropriate only if both sides agreed to a cease-fire and wanted a U.N. force to help police a truce.

{In Washington, State Department spokeswoman Phyllis Oakley said the United States was "certainly prepared to listen to any ideas" the Soviets had regarding a U.N-flagged naval force but added that to date their proposals had been "imprecise."}