China has informed the U.N. Security Council that it is ready to support a U.S.-backed resolution calling for an immediate cease-fire in the Iran-Iraq war and the opening of negotiations to end the conflict, a senior administration official indicated yesterday.

Under Secretary of State for Political Affairs Michael H. Armacost, appearing on the CBS program "Face the Nation," said all five permanent members of the U.N. Security Council now agree on the first of two resolutions the United States is sponsoring in a bid to force Iran into negotiations.

Armacost said the Reagan administration has found, "to our surprise," that "all of the permanent members" are backing the resolution. "The question now is whether we can negotiate agreement on mandatory sanctions," he added referring to the thrust of the second resolution.

Another U.S. official, who asked not to be identified, called China's decision to back the resolution "a major achievement" for the U.S.-led campaign at the United Nations to pressure Iran in particular to end the war. "They were the last holdout" among the permanent Security Council members, he said.

But he said that the administration does not want a vote on the first resolution unless the sanctions resolution is also approved to avoid passing another meaningless U.N. resolution.

The administration learned of the Chinese decision almost two weeks ago, Armacost said. There was a long period of debate in the Chinese government about what position to take. China has become a major arms supplier to Iran, including selling it Silkworm missiles that U.S. officials said threaten shipping in the Persian Gulf.

China has repeatedly denied it is selling arms to Iran.

Aramcost indicated there is no agreement, however, on the second, more controversial U.N. resolution that would impose mandatory sanctions, including an arms embargo, on whichever country, if any, refused to comply with the terms of the first resolution.

Iraq has said it is ready to comply with the terms of the first resolution. But Iran continues to refuse an end to the war while Iraqi President Saddam Hussein is in power.

The administration believes it has the support of Britain and France for the second resolution, but is uncertain whether the Soviet Union or China will support it. Together with the United States, these four countries make up the permanent members of the U.N. Security Council. Any of them could veto the imposition of mandatory sanctions.

In addition to a cease-fire and negotiations, the first resolution also calls on Iran and Iraq to withdraw their respective forces to the borders internationally recognized before the start of the conflict in September 1980.