The United States and its principal allies are prepared to impose economic sanctions of their own against Iran if the Soviet Union vetoes formal sanctions by the United Nations, a senior State Department official said late yesterday.

Speaking to reporters at the State Department on condition that he not be quoted by name, the high-level official said the possibility of proceeding despite a Soviet veto has been discussed with major allies. "I expect we will have support," he declared.

The official suggested without giving details that the sanctions under such a makeshift arrangement would be the same ones that would be imposed by U.N. vote unless, as appears increasingly likely, Moscow says no.

While conceding that this arrangement would be less effective than formal U.N. sanctions, the official expressed confidence that the measures would be strong enough to be felt by Iran.

Asked if the economic sanctions might be followed by a naval blockade against Iran, the official said, "I am not ruling out anything."

Secretary of State Cyrus R. Vance, briefing more than 40 former high government officials at the White House yesterday morning, said the United States has obtained at least eight of the nine votes necessary for Security Council approval of a sanctions resolution against Iran. However, the Soviet Union, as one of the five permanent members of the council, can kill any measure with a veto.

In a briefing for reporters last night, the high-level State Department official said the number of militants holding the American hostages in Tehran appears to have grown considerably in the nine weeks of captivity. About 600 captors are believed to be involved, he said.

The United States has information that some captors have received training from the Palestine Liberation Organization. However, he said there is no information that Palestinians are among the captors.

Beyond economic sanctions, the United States has some additional options short of military action that could be exercised to press Iran for release of the hostages, reporters were told. However, the official refused to be specific.

Regarding the Soviet invasion of Afghanistan, the State Department official said U.S. policy now has two main objectives:

To make clear to the Russians that they will incur a substantial cost for their continued presence in Afghanistan.

To warn that similar military action elsewhere will involve "a very heavy cost." The official emphasized the word very but did not spell out what the costs might be.

The official declined to speculate on how or why the Soviets decided to "cross a new threshold" by sending their troops into Afghanistan, except to remark that neither President Leonid I. Brezhnev nor Premier Alexei N. Kosygin is playing the large leadership role they took in the past, evidently because ill health makes this impossible.

The Soviet action in Afghanistan has brought together the United States and the People's Republic of China, the official said, in recognition of the threat posed for Southwest Asia and the resolve to supply Pakistan, which borders Afghanistan, with military assistance. However, tandem action by Washington and Peking to counter the Soviets in Afghanistan falls short of a worldwide Sino-American alliance, reporters were told.