TEHRAN, AUG. 20 -- Iran acknowledged for the first time tonight that it has placed mines in the Persian Gulf, but insisted that the explosive devices were planted only to protect Iranian coastal installations and did not inflict damage to oil tankers recently struck by mines in the Sea of Oman and on the Arab side of the gulf.

"As long as there are foreign forces in the gulf, it is quite natural to use such means to block approaches," said Kamal Kharazi, spokesman for the Iranian Supreme Defense Council and head of its War Information Center. "But use of mines by Iran was not designed to block freedom of navigation."

He added: "We believe there are different kinds of mines in the Persian Gulf. Some may be U.S. mines, some may be Iraqi mines and some may be Iranian mines. In order to defend ourselves, we use mines."

Kharazi, believed to speak with the full authority of Iran's Islamic leadership, thus provided the first Iranian explanation of its role in the gulf mining that has provoked the major buildup of U.S. and European naval forces and raised tensions to a dangerous level in the critical waterway.

{Iran's speaker of parliament, Ali Akbar Hashemi Rafsanjani, meanwhile raised the prospect that eight American hostages held in Lebanon could be released if Shiite prisoners held in Kuwaiti and Israeli jails were freed in exchange.

{Rafsanjani, speaking in an interview with NBC News, said Iran is prepared to use its influence with Lebanese Shiite captors holding the eight Americans if the United States puts pressure on Israel and Kuwait to release Shiite Moslems incarcerated in their countries on terrorist charges.

{The State Department firmly rejected the Iranian proposal. Spokeswoman Phyllis Oakley said, "Our response to Mr. Rafsanjani is: No deals. The United States will not make concessions to terrorists nor will we ask others to do so. Making concessions only encourages additional acts of terrorism."

{In Israel, Prime Minister Yitzhak Shamir, asked about the Iranian statement, said Israel will study the proposal. "I must say there is not a general policy about this issue," he said. "Our way is to make a judgment on every case on its specific characteristics."}

While Kharazi denied that Iranian mines are aimed at commercial shipping, his acknowledgment that Iran has planted some in the gulf reinforced earlier warnings by high Iranian leaders that ships bound for Arab ports will become major targets if Iraq resumes attacks on oil tankers moving into and out of Iranian ports.

Kharazi did not say exactly where Iran has sown mines. But he depicted the deployment as a protection of Iranian ports and facilities on the Iranian side of the gulf.

In response to questions, Kharazi specifically denied Iranian responsibility for mines that have hit commercial vessels in the Sea of Oman off Fujayrah and in gulf shipping lanes leading to Kuwait.

Western governments have expressed the belief that small Iranian craft laid the mines. The Reagan administration formally accused Iran of planting a mine that hit a U.S.-operated supertanker, the Texaco Caribbean, off Fujayrah on Aug. 10. The vessel, of Panamanian registry, was carrying a load of Iranian crude when it was damaged.

Some U.S. and European officials have suggested that Iran's Revolutionary Guards and their irregular navy of small speedboats could have laid the mines, perhaps acting out of revolutionary zeal and exceeding orders from Tehran.

Noting the military buildup in the gulf, these officials also have warned that a Revolutionary Guard unit could generate hostilities by attacking a U.S. or European ship, perhaps using unorthodox tactics or firing a missile from a small boat.

Kharazi acknowledged that Iran has "classical" and "nonclassical" forces in the gulf area, but he insisted that the Revolutionary Guards act only on orders from the leadership in Tehran.

Diplomatic sources in Tehran also expressed doubt that Revolutionary Guard forces act on their own in the gulf or on Iran's other war fronts. "I think those decisions, the mining and everything, are taken right here in Tehran," one diplomat said.

Kharazi accused the United States and Iraq of increasing tension in the gulf through the growing U.S. naval force. He said the mines found off Fujayrah and in Kuwaiti shipping lanes were planted there by "the United States or its agents," a reference to Iraq.

Iran, he declared, has sought only the right to ship its oil peacefully out of the waterway. "We shall remain on the defensive as long as we are not attacked," he said.

President Saddam Hussein of Iraq was reported this week to be reserving the right to renew attacks on Iranian oil shipments to weaken the economy here and diminish Iran's ability to continue its seven-year-old struggle against Baghdad. According to reports here, however, the United States has urged Saddam Hussein to refrain from such attacks because of the explosive gulf situation.

In Iranian eyes, Saddam Hussein's decision to begin air attacks on Iranian oil shipments four years ago created the dangerous situation today.

Kharazi, reflecting that assessment, said Iran has no objection to U.S. escort operations for Kuwaiti tankers moving in and out of the gulf under U.S. flags.

"We are not opposed, in order that Kuwaiti oil may be exported," he said, in a clear attempt to draw a parellel with Iran's need to export its own oil. At the same time, Kharazi said the presence of U.S. warships in gulf waters is responsible for the increased tension and fears of armed conflict.

Tehran has insisted that the Iranian Navy has enough equipment and skill to rid shipping lanes of any mines to guarantee safe passage.