President Reagan said last night that the United States has not paid "ransom" to Iran for the American hostages in Lebanon, but has covertly sent arms to Tehran to gain "access and influence" there, end the 6-year-old Iran-Iraq war and stem international terrorism.

In a nationally televised address from the Oval Office, Reagan defended the "secret diplomatic initiative" against rising criticism from Congress and abroad that he violated his own policy against negotiating with terrorists in a bid to buy freedom for the Americans being held in Lebanon.

"We did not -- repeat, did not -- trade weapons or anything else for hostages -- nor will we," Reagan said. "Those who think we have 'gone soft' on terrorism should take up the question with [Libyan] Colonel [Moammar] Gadhafi."

Claiming that the Iran operation was begun 18 months ago "for the best of reasons," Reagan acknowledged that U.S. officials had talked with unidentified factions in Iran about pressuring other groups in Lebanon to release the American hostages. But he denied that the arms sent to Iran were a "ransom payment."

White House chief of staff Donald T. Regan said in an interview yesterday that the president authorized "exploratory" contacts with Iran in hopes of curbing its role in international terrorism and opening avenues to moderate factions there. Regan said the arms shipments were made later as a demonstration of "good faith" to these factions, and that the United States asked the Iranians to "use their influence on people to get the hostages released."

Regan added that "occasionally their influence worked" and produced the release of the Rev. Benjamin Weir, the Rev. Lawrence Martin Jenco and David P. Jacobsen. "Did the arms shipments coincide with this? No. As the Iranian ambassador said the other day, sometimes they did and sometimes they didn't. They were not being sent for that purpose."

In his address, Reagan said the United States "has not made concessions" to the demands by extremist groups in Lebanon holding Americans hostage that 17 terrorists imprisoned in Kuwait be released.

Reagan discussed only the broadest details of the Iran operation. He did not mention strong objections that were raised to the shipments of arms to Iran by Secretary of State George P. Shultz and Secretary of Defense Caspar W. Weinberger.

He also omitted mention of Israel's role in establishing contacts inside Iran and shipping the U.S. weapons, and he sidestepped the question of whether the timing of the arms shipments was linked to release of the hostages, as other officials have reported.

Instead, Reagan said that the arms shipped were "small amounts" and "modest deliveries" that "could easily fit into a single cargo plane." He said the weapons were "defensive" in nature and "my purpose was to convince Tehran that our negotiators were acting with my authority, to send a signal that the United States was prepared to replace the animosity between us with a new relationship."

While Reagan avoided details, other officials said the arms may have included surface-to-air missiles, antitank weapons and spare parts, especially for Iran's force of U.S.-built F14 jet fighters. Some of the weapons may have been sent by Israel and not directly from the United States. The president said that the weapons "could not, taken together, affect the outcome of the six-year war between Iran and Iraq."

The administration had long been on record as favoring neutrality in the war and has called on other nations to join in an arms embargo of Iran. A senior White House official, speaking to reporters at a briefing yesterday, said Reagan secretly authorized "some specific waivers of that embargo" to send the weapons to Iran.

"The amount of material that was shipped was minuscule," the senior official said, adding that it "had absolutely no effect on the balance of the war. It was . . . a demonstration of good faith and an indication that the people we were dealing with could gain some support from the United States."

"Now we don't want unlimited shipments from the United States, we don't want unlimited shipments from Germany, from France, from China. And we have worked to try and prevent that," the official added. "We will admit that we were not totally successful; arms still get through. But we don't want an uncontrolled supply to fuel the war because we're trying to bring it to an end."

Asked why the president had apparently broken his principle against sending the weapons to Iran, the senior official said: "We have never said that we weren't shipping arms to Iran. That would be the matter of principle if we'd said we'd never shipped any. We haven't commented on it. At the same time, we've worked to stop uncontrolled shipments. I don't see anything inconsistent with that at all."

The senior official said the United States had used "unconventional methods" in a gesture to a faction in Iran that he described as "moderate" and interested in closer ties with Washington. He said the arms shipments were designed to give some "credibility" to the moderate elements.

The senior official said the Iran operation was approved by Reagan in a directive signed in January, although discussions about it occurred before that point. The official refused to provide details on the number of shipments to Iran on grounds that "the radical elements will be able to figure out who was dealing with the United States." At first, the official said no shipments were made before Reagan approved the Iran operation in January, but he then said one shipment was made "in our interest" by another country before that, about the time Weir was released in September 1985.

Another senior White House official, speaking in an interview, said Attorney General Edwin Meese III had provided a written legal opinion that the Iran operation did not violate any laws.

In his address, Reagan said, "Those with whom we were in contact took considerable risks and needed a signal of our serious intent if they were to carry on and broaden the dialogue."

When the secret effort was started, Reagan said, "We made clear that Iran must oppose all forms of international terrorism as a condition of progress in our relationship. The most significant step which Iran could take, we indicated, would be to use its influence in Lebanon to secure the release of all hostages held there.

"Some progress has already been made," the president said. "Since U.S. government contact began with Iran, there has been no evidence of Iranian government complicity in acts of terrorism against the United States. Hostages have come home -- and we welcome the efforts that the government of Iran has taken in the past and is currently undertaking."

The senior official who briefed reporters said that U.S. officials believed they had an "agreement" with Iran to curb international terrorism and the kidnaping of Americans. The official said this agreement lasted until the most recent spate of hostage-taking.

Separately, chief of staff Regan said in the interview that Ayatollah Ruhollah Khomeini had issued a document on terrorism and added that "we have no knowledge of any terrorist act by Iran" or groups it influences since the U.S. contacts began.

However, during the entire period the administration kept Iran on its list of nations sponsoring terrorism. Moreover, Reagan denounced Iran for its support of terrorist activities at about the same time the United States began its effort to make contacts with Iran. In a speech to the American Bar Association on July 8, 1985, the president said Iran was among a group of nations -- including Libya, North Korea, Cuba and Nicaragua -- that he identified as "Murder Incorporated."

"Well, yes, only recently the prime minister of Iran visited Nicaragua bearing expressions of solidarity from the ayatollah for the Sandinista communists," he said.

While the administration had previously cast Iran as an "outlaw" nation, last night the president declared that it is in the "national interest" of the United States "to watch for changes within Iran that might offer hope for an improved relationship." He said there was little hope "until last year" when intermediaries suggested a "direct dialogue with Iranian officials."

Reagan noted that Iran lies "between the Soviet Union and access to the warm waters of the Indian Ocean." He said, "Geography explains why the Soviet Union has sent an army into Afghanistan to dominate that country and, if they could, Iran and Pakistan." The president also noted the importance of Iran's oil reserves to the world economy.

"Without Iran's cooperation we cannot bring an end to the Persian Gulf war; without Iran's concurrence, there can be no enduring peace in the Middle East," Reagan said.

Administration officials said there has been continuing disagreement in the White House over how and when to disclose the Iran operation to Congress and the American public. While chief of staff Regan urged disclosure, national security affairs adviser John M. Poindexter advocated continued secrecy.

Last night, Reagan acknowledged that information about the Iran operation was limited to top U.S. officials. He said Congress would now be informed, and did not contest reports that the White House had purposely not informed congressional leaders previously. The senior White House official who briefed reporters said the law allows a president in such situations to tell Congress after the fact of such an operation.

The president has not held a news conference since August, and he skirted many questions about the Iran operation in his address last night. However, he opened his remarks by attacking what he said were false news reports about the operation.

He branded as "utterly false" reports suggesting an arms-for-hostages deal with Iran. Noting other reports about a possible Danish sealift to Iran, secret U.S. shipments through Spanish and Italian ports, and U.S. shipments of spare parts for combat aircraft, Reagan said, "All these reports are quite exciting; as far as we are concerned, not one of them is true."

Although Reagan chastised American and world press reports about the Iran operation, he did not mention that the initial report came from a disclosure made by one faction in Iran opposed to contacts with the United States. The disclosure was made to a pro-Syrian Lebanese magazine, which reported that former national security affairs adviser Robert C. McFarlane had traveled to Tehran for talks with officials there.

Presidential aides have acknowledged that the Iran operation was "undone" by the infighting among Iranian factions.

The senior officials said yesterday that McFarlane made only one visit to Iran. The pro-Syrian magazine, Ash Shirra, reported yesterday that McFarlane visited Tehran twice to offer arms to Iran.

Reagan said McFarlane's mission was intended to "open a dialogue," and that progress has been made since then.

Meanwhile, the senior official who briefed reporters said that the administration will have to find "some other way" to seek release of the six remaining American hostages because of "damage" caused by the disclosure of the secret operation.