National security adviser Frank C. Carlucci said yesterday that the United States cannot afford to back down on its offer to provide military protection in the Persian Gulf to threatened Kuwaiti oil tankers.

Echoing comments by President Reagan in a nationally televised address last night, Carlucci warned in a speech prepared for delivery to the National Association of Arab Americans last night that any retreat would invite the Soviet Union and Iran to fill the vacuum in the gulf and would amount to a major defeat for U.S. foreign policy.

"We cannot afford to let hostile powers -- either the Soviets or the ayatollah {Ruhollah Khomeini} -- gain a chokehold on so central a region," he said in the speech. "The president understands that's at stake, and like seven presidents before him, is determined to prevent it."

"He knows the consequences of retreat from our offer to reflag Kuwaiti ships," Carlucci added in his remarks.

The United States' allies in the gulf, he said, "will be faced with either giving in to Iranian intimidation or accepting Soviet offers of protection, and not just for shipping."

Carlucci's speech came on the eve of a Senate debate of a bill that would prohibit Kuwait and any other gulf nation from placing ships under U.S. flag protection. The White House yesterday sent Congress classified and unclassified versions of a report on arrangements the United States has made for protecting 11 Kuwaiti oil tankers.

"Would our congressional and press critics have us fall back exclusively on diplomatic demarches, stop helping our friends defend themselves and stop protecting U.S. flag vessels?" Carlucci said. "Are we to say to Kuwait -- a neutral state -- that its ships are unacceptable because we have been threatened by Iran?"

Carlucci said the United States is not abandoning its neutrality in the Iran-Iraq war by escorting the reflagged tankers of Kuwait, a close ally of Iraq, because Washington still will not supply arms to either side. The reflagged Kuwaiti ships will not carry war materiel, he added.

The executive summary of the unclassified White House report does not address the issue of what the United States would do if U.S. Navy escort ships or the tankers flying U.S. flags are threatened by Iranian Silkworm land-to-sea missiles.

The summary said that U.S. forces will use "proportional force as necessary in self-defense" to deal with any attacks. "This right will be exercised in the face of attack or hostile intent indicating imminent attack," it said.

The report said that the planned addition of three warships to the U.S. Middle East Force now stationed in the gulf will be sufficient to "meet potential sea, air and land based missile threats." The report made no mention of any need for landing rights for U.S. warplanes in gulf nations, a development some officials believe is important.

Carlucci and the White House report argued that America's Western and Arab gulf allies are already providing the United States with substantial political and military support.

The British, Carlucci said, already have a higher proportion of their navy committed to the gulf and Indian Ocean than does the United States. The British have provided protection for "over 100 ships" since January, "which is more than we have protected," he added.