The United States said yesterday it is "extremely disappointed" that Britain has backed down on its promised economic sanctions against Iran and expressed concern that the action could undermine efforts to put Iran under concerted pressure from America's major allies.

Speaking on the 200th day of captivity for the American hostages in Iran, State Department spokesman Hodding Carter complained that the British had indeed "reversed" themselves, and that, he added, "goes to the question of cohesion" within the Western alliance.

The sanctions measures, originally agreed to by the nine members of the European Economic Community, were watered down over U.S. protests when the EEC foreign ministers met in Naples last week.

Then, on Tuesday, Prime Minister Margaret Thatcher's government, faced with a possible revolt in the British Parliament, retreated even further. The EEC had agreed to make the sanctions retroactive to Nov. 4 but Britain said it would bar only furture sales to Iran.

In response, Hodding Carter echoed the line laid down Tuesday by Secretary of State Edmund S. Muskie that, despite U.S. disappointment over all the dilutions, the sanctions still will have an economic impact on Iran and are "bound to be useful. . . ."

But in a pointed reference to Britain and concern that its action could lead other allies to find domestic political reasons for softening their support of sanctions still further, the spokesman added:

"It is not useful when nations, in effect, say to Iran that a basic violation of international law by Iran is not of sufficient concern to produce a unified front by the international community."

The criticism of Britain marked the second time in as many days that the United States has publicly complained about the actions of a major European ally and touched off intense new speculation as to the state of U.S.-West European relations under the strain of the twin crises in Iran and Afghanistan.

At a Tuesday news conference, Muskie scolded France sharply for failing to consult the United States or its other allies in advance about President Valery Giscard d'Estaing's meeting in Warsaw Monday with Soviet President Leonid I. Brezhnev.

That led French Foreign Minister Jean Francois-Poncet to reply yesterday that France did not require "prior approval" from Washington to deal with the Soviets.

In response, U.S. officials said yesterday that the French retort was irrelevant to Muskie's complaint. The United States, they noted, has no intention of vetoing French diplomatic initiatives; but, they added, in light of past French complaints of U.S. failure to consult, Washington feels it has a right to be informed of French moves and have an opportunity to discuss them within the context of their effect on concerted western strategy.

Further strains have been caused in U.S.-West European relations during recent days by the decisions of the national Olympic committees in several European countries to ignore the recommendations of their governments to boycott the Olympic Games in Moscow this summer. That has severely damaged U.S. hopes of using an Olympic boycott to symbolize international disapproval of the Soviet invasion of Afghanistan.

Commenting on the tensions and disputes that have cropped up lately, Hodding Carter noted that throughout the 30-year history of the North Atlantic Treaty Organization, the United States and its NATO partners frequently have been at odds over how to deal with situations outside NATO's immediate province of European and Atlantic defense.

He cited past acrimonious transatlantic disputes over the Vietnam war and and approaches to China, Cuba and the Middle East, and said: "They've been a regular feature as long as we've had an alliance. To forget that is to forget your history."

But, he added, that does not mean that the current disputes within the alliance should be "minimized" or are not "matters of concern." He said, "I'm in no way trying to suggest that we are not concerned about some of the steps that have been taken in recent days."

In contrast to the clear annoyance being expressed about many allies, the State Department had strong praise yesterday for one old U.S. friend, Australia, which decided Tuesday to impose a trade embargo against Iran.

Hodding Carter noted that the Australian embargo would follow the lines the United States had wanted the West Europeans to adopt -- namely, that it will cover all existing and future contracts for sales to Iran except for food and medicine.

That, the spokesman noted, will affect the sale of several million dollars worth of Australian steel, iron and wool to Iran. He added that Muskie personally had thanked the Australian Ambassador here, Sir Nicholas Parkinson, for his government's support.