The United States will keep its 27-ship naval force in the Persian Gulf at full strength until the Iran-Iraq ceasefire, in effect since Saturday, is clearly holding, U.S. officials say.

The Reagan administration also dismissed as a familiar ploy a Soviet government offer Sunday to withdraw its ships from the gulf if the United States and its allies do likewise.

Although a Pentagon spokesman said he knew of no violations of the cease-fire in the gulf, "We think it's prudent to ensure that the cease-fire is holding" before thinning out the U.S. flotilla.

The original mission of the American flotilla _ to escort U.S.-flagged ships in the gulf _ was broadened by President Reagan in April to include ships of all friendly nations requesting protection.

Britain, France, Italy, the Netherlands and Belgium, all U.S. NATO allies, will also continue to keep their ships on escort and mine-sweeping duty, according to spokesmen at their embassies here.

Britain has nine vessels in the gulf or adjacent waters in the gulf of Oman. Italy has six, France two and the Netherlands and Belgium one each.

Even though the attacks have stopped, said a Dutch diplomat, "There are still mines to be swept."

The Soviet proposal, that all warships _ theirs, U.S. and allied _ pull out of the gulf and leave peacekeeping to the United Nations "tracks with what they've been saying off and on for a year and a half," said a State Department official.

A Defense Department official said, "We've had a continuous presence in the gulf since the late `40s. We've been up-front with the Soviets about our interests in the region." The United States normally posts five ships in the gulf.

Although the Soviet gulf operations have attracted scant attention, a U.S. Navy spokesman said the Soviet force in the gulf consists of two or three minesweepers, two guided-missile destroyers and one or two guided-missiles frigates, plus several auxiliary vessels.