The United States yesterday condemned the Israeli attack on Iraq's nuclear reactor and said Israel may have violated U.S. law by improperly using American-supplied aircraft to carry out the raid.

In a statement read on behalf of the White House and the State Department, department spokesman Dean Fischer said: "The United States government condemns the reported Israeli air strike on the Iraqi nuclear facility, the unprecedented character of which cannot but seriously add to the already tense situation in the area."

He added, "Available evidence suggests U.S.-provided equipment was employed in possible violation of the applicable agreement under which it was sold to Israel, and a report to this effect is being prepared for submission to the U.S. Congress in accordance with the relevant U.S. statute."

The Foreign Military Sales Act bars use of American weapons supplied to other countries for "purposes other than national or regional self-defense." If Israel is found to have violated this provision, it could face suspension of the massive military aid it receives from this country, although the Reagan administration is unlikely to take such action.

Instead, yesterday's U.S. response appeared to be what various administration sources called "a warning shot" aimed at reining in what U.S. officials privately regard as the increasingly bellicose and reckless conduct of Prime Minister Menachem Begin's government, which is engaged in a tough battle to retain power in Israeli national elections later this month.

The officials expressed particular concern that the Israeli action will prod the feuding governments of the Arab world to unite in at least rhetorical support of Iraq. If that happens, the officials said, the resulting atmosphere of tension and invective could cripple the efforts of U.S. special envoy Philip C. Habib to work out an agreement ending the confrontation between Israel and Syria over Syrian missiles in Lebanon.

Habib has been preparing to resume his mediation efforts, and Fischer yesterday stressed the U.S. hope that his efforts will continue and that "the other parties share our sense of the mission's continuing importance."

One State Department official characterized the U.S. reaction as "close to the maximum that we could do, given the realities of our ties and commitments to Israel." He said the statement of condemnation was intended to show the Arab states "in unequivocal terms" that Washington does not condone Israel's action.

The official, who declined to be identified, said the United States was making an issue out of the use of American planes to remind Israel that it is dependent for its defense needs on U.S. weaponry and that irresponsible use of this equipment could produce reactions in this country that would make Congress view future Israeli arms requests with a tougher, more skeptical eye.

Although Fischer refused to discuss what U.S. equipment might have been involved, other administration sources said privately that the Israelis are believed to have used F4 Phantom jets to carry out the raid. These sources also said that, despite Iraqi claims about the raid being a failure, there was no doubt that the reactor, located near Baghdad, had been demolished.

There is strong belief in U.S. and other diplomatic circles that last December an Israeli Phantom jet, using as cover the fighting between Iraq and Iran, attacked the installation, damaging the main building.

In response to questions about Israeli charges that Iraq was preparing nuclear weapons for possible use against the Jewish state, Fischer said Iraq is a signatory to the nuclear nonproliferation treaty, that it has agreed to accept the International Atomic Energy Agency's safeguards for detecting work on nuclear bombs and that the United State has no evidence of nuclear-weapons building by Iraq.

Fischer added, though, that the United States had been concerned that the Iraqi reactor "could pose a risk of nuclear proliferation at some point." Other U.S. officials privately went further, saying that U.S. intelligence estimates indicated that the facility would have become operational shortly and would have been capable of producing weapons-grade material.

The Israeli government contends that if it had waited for the reactor to go into operation, Israel might have been precluded from knocking it out then because of the danger that an attack would produce severe nuclear effects in the atmosphere. Fisher said the U.S. estimate of the radiation effects from the Sunday raid was that "they probably would be minimal and limited to the immediate vicinity of the installation."

The United States, he added, "is ready to respond to any requests for help in monitoring the extent of any nuclear effects and in dealing with any other related problems."

Fischer and other administration officials went to great lengths to stress that the United States had no advance knowledge of the raid. They said the U.S. Embassy in Tel Aviv was informed by the Israeli government Sunday shortly after the raid and that word was passed immediately to Secretary of State Alexander M. Haig Jr., who relayed it to top administration officials and the congressional leadership.

According to White House sources, President Reagan was informed Sunday at Camp David in a telephone call from from his national security adviser, Richard V. Allen. The sources said that as of last night, the president, who was described as "surprised" at the news, had made no attempt to contact Begin directly, although Haig talked with Israeli Ambassador Ephraim Evron by phone Sunday and yesterday. The sources refused to specify what was said.

On Capitol Hill, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) called the raid "a cause for great concern" and expressed fear that it could seriously damage the Habib mission. A similar comment came from Rep. Clement J. Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, who called the raid "a highly irresponsible act" that apparently violated U.S. law.

However, Sen. Alan Cranston (D-Calif.), who warned last March of Iraqi ability to build a nuclear bomb, called the strike "inevitable," said it was debatable whether U.S. law was violated and added that he would neither "condemn nor condone" it.