The United States Navy clashed with Iranian forces across the southern half of the Persian Gulf today, crippling or sinking six armed Iranian vessels. One American attack helicopter was reported missing.
The attacks began when six American ships destroyed two Iranian oil platforms in what the Reagan Administration said was retaliation for the mining that damaged a Navy vessel last week.
The worst fighting between Iranian and American forces coincided with heavy clashes in the Persian Gulf war between Iran and Iraq. In that land battle, Iraqi forces assaulted the Iranian-held Fao Peninsula at the northern end of the gulf. Iraq maintained that its forces were retaking the small peninsula, which has been in Iranian hands for two years. [ Page A11. ] U.S. Denies Aiding Iraq Iran has been able to use the peninsula as a launching base for Chinese-made Silkworm missiles aimed at Kuwait. It has also used its presence there to threaten Basra, the key Iraqi city near the Iranian border.
Iran asserted that American helicopters were aiding Iraq in the Fao Peninsula attack, but United States officials, including Defense Secretary Frank C. Carlucci 3d, said this was not true.
The State Department's chief spokesman, Charles E. Redman, said the United States did not know of Iraqi actions at Fao beforehand, nor did it notify Iraq in advance of its intended strikes in the gulf. ''There is no linkage or connection between these two events,'' he said. Heavy Iran Casualties Reported.
The attacks on the Iranian oil platforms, ordered by President Reagan, set off a stepped-up series of attacks and counterattacks..
Iranian casualties were believed to have been heavy. The official Iranian press agency said there were deaths and injuries aboard the oil platforms attacked, but it gave no numbers.
Late today, the Pentagon said that one American Cobra helicopter was missing with its two-man crew. The Iranian press agency said one American helicopter had been shot down by Iran's navy. 'No Illusions,' Reagan Warns
''We've taken this action to make certain the Iranians have no illusions about the cost of irresponsible behavior,'' President Reagan said today. ''We aim to deter Iranian aggression, not provoke it.
''They must know that we will protect our ships, and if they threaten us, they'll pay a price,'' he said.
Congressional leaders, who were consulted by the White house before the military action began last night and kept closely informed today, largely supported President Reagan's action and called it a justified response to Iran's renewed mining off the gulf.
''There will be no fight as long as they keep consulting,'' said Representative Tony Coelho of California, the House Democratic whip, who has been a frequent critic of the Administration's foreign policy.
The clashes between Iranian and American ships and planes occurred at several locations, and expanded as Iranian forces attacked or threatened American warships, an oil tanker flying the British flag, an oil platform operated by an American company in the Mubarak oilfield off Sharjah, and a commercial supply ship flying the American flag. Heaviest Fighting to Date
The Defense Department indicated that the British-flag tanker had been severely damaged but could not provide details. Damage to the American-operated oil platform was believed to be light.
The Navy responded to these attacks with missiles and bombs fired from ships and aircraft, in the most intense fighting to date between the two nations.
Military officials said they were startled at the vigorous opposition that the Navy met from Iranian forces, which repeatedly moved ships and aircraft against American forces despite being heavily outgunned. The United States, the officials said, had expected little resistance to what was designed as a measured response to Iran's laying of mines in the gulf, where an American frigate hit a mine on Thursday. Congressional Leaders Advised
President Reagan on Sunday night ordered the initial strikes, which began at about 1 A.M. Eastern time today, officials said. He informed Congressional leaders of his decision at a White House meeting also on Sunday night.
At the United Nations shortly after noon today, the deputy American representative, Herbert S. Okun, informed the president of the Security Council, Peter D. Zuze of Zambia, of the clashes with Iranian forces.
Mr. Okun told Mr. Zuze that the attacks were acts of self-defense permitted under Article 51 of the United Nations Charter, American diplomats said.
As night fell in the Persian Gulf, Mr. Carlucci appeared at his second press conference of the day to declare that ''our initial mission has been accomplished.''
By JOHN H. CUSHMAN JR. and SPECIAL TO THE NEW YORK TIMESAPRIL 19, 1988