Israel branded as "unjust" today the Reagan administration's suspension of delivery of four F16 fighter aircraft to the Israeli Air Force, insisting that Israel acted in self-defense in bombing an Iraqi nuclear reactor near Baghdad.

The Israeli government "had tried to utilize every diplomatic avenue which was open, and only after the efforts failed, Israel was forced to take the action it took," according to a statement issued by the Foreign Ministry. tThe statement said, "We very much regret the decision of the U.S. administration to suspend delivery of aircraft to Israel."

Also, Israeli Deputy Defense Minister Mordechai Zippori, responding to reporters' questions, said U.s.-built F16s and F15s were used in Sunday's bombing raid because the mission was planned for aircraft that Israel deemed "most suitable" from a military standpoint.

"The definition of Israel's defense will be decided by Israel's government and not by any other government, no matter how friendly," Zippori said.

Prime Minister Menachem Begin said tonight that the Israeli raid destroyed a secret installation 120 feet below the ground that had been built to evade detection by the International Atomic Energy Agency. He gave no details about the purpose of the installation.

Naftali Lavie, public affairs adviser to Foreign Minister Yitzhak Shamir, said Israeli intends to launch a lobbying campaign in Washington, covering Capitol Hill, the administration and the news media in an effort to persuade Congress to approve the delivery of the withheld aircraft.

Obviously stung by the first U.S. suspension of arms shipments to Israel and the accompanying rebuke by Secretary of State Alexander M. Haig Jr., Israeli officials sought to portray the air strike against Iraq's French-supplied 70-megawatt reactor as a last-resort measure taken only after every other effort to head off Iraq's acquiring nuclear weapons capability had failed.

Lavie said that since 1975, Israel had tried through diplomatic channels to persuade France not to sell reactors and enriched uranium to Iraq.

When asked why Israel, after devoting five years to trying to persuade the government of former president Valery Giscard d'Estaing, gave only two weeks to the new government of President Francois Mitterrand before bombing the reactor, Lavie said, "Because before, we were not yet at that dangerous point to which we came in the last few weeks."

He was referring to Begin's contention that after the installation of uranium in the core of the Iraqi reactor, possibly early next month, bombing it even with conventional explosives would have set off a chain reaction that could scatter deadly radiation over densely populated Baghdad, only 12 miles away.

Also, Lavie said, Israel made no distinction between the governments of Giscard and Mitterrand. "Our position was known by the French government," he said.

When it was pointed out that Mitterrand, as an opposition leader and presidential candidate, had indicated on several occasions a reluctance to equip Iraq and other Middle East states with nuclear weapons capabilities, Lavie replied, "For us, it is a continuous government. We continued our approach without distinction." Lavie would not say when and how many times the new French government was approached, saying only that contact was through embassies in Tel Aviv and Paris.

In any case, Lavie argued, time was running short for Israel before the reactor would become "hot" and military action was imperative.

Lavie paraphrased an Oct. 22, 1962, speech by John F. Kennedy, during the Cuban missile crisis, saying, "We live in a world in which not only pulling the trigger is an imminent danger to war."

Zippori said Israel's use of the F16s and F15s did not violate a 1952 agreement restricting U.s.-supplied weapons to defensive use. He said the Reagan administration's decision yesterday was a result of a "misunderstanding" and added that he doubted that it will lead to a real crisis in relations between the two countries.

The United States has already delivered 53 of the 75 F16s it promised to sell Israel, as well as 25 of the promised 40 F15 fighters.

Zippori said it has always been clear that Israel does not need U.S. weapons for display at military parades and that there always has been an understanding of the significance of U.S. military aid to Israel.

His remark apparently was an oblique reference to the lack of serious protest by the United States when U.s.-supplied jets, including the F16, have been used repeatedly in preemptive strikes against Palestinian guerrilla positions in Lebanon.