President Reagan stepped up the U.S. military role in the Persian Gulf on Friday, ordering the Navy to begin protecting neutral merchant ships that fall under attack and request help.

The action came 11 days after American warships and aircraft clashed with Iranian forces in the gulf, sinking or disabling six Iranian vessels.Although the new policy was believed aimed at Iran, the United States said it should not be viewed "as a tilt in either direction" in the tanker war between Iran and Iraq.

Announcing the policy, Defense Secretary Frank Carlucci said, "We are not the policeman of the gulf, nor do we wish to be.

"For over 200 years, U.S. policy has been to help protect freedom of navigation in international waters. This (new) assistance is a logical and humanitarian outgrowth of recent events in the gulf which further strengthens our adherence to this principle," Carlucci said.

"We cannot stand by and watch innocent people be killed or maimed by malicious, lawless actions when we have the means to assist, and perhaps prevent them," he added. "We do not intend to describe our specific rules of engagement or the methods we plan to use in rendering this assistance."

The Marine Corps, meanwhile, declared that two officers who were listed as missing after the fighting 11 days earlier were "killed in action" and disclosed that they had reported a hostile radar had locked on their helicopter gunship before it disappeared.

In a statement, the corps refused to say the AH-1T Cobra gunship flown by Capt. Stephen C. Leslie and Capt. Kenneth W. Hill had definitely been shot down by Iranian forces, as Iran has claimed.

Reagan's action, likely to be seen by critics as drawing the United States deeper into the Iran-Iraq war, extends Navy protection to ships that come under attack and request assistance.

For years, the Navy has protected U.S.-flagged ships in the gulf. Last summer, 11 Kuwaiti oil tankers were allowed to reregister as U.S. flag vessels to secure that protection.

In expanding the protective umbrella, Carlucci said, "Such aid will be provided to friendly, innocent neutral vessels flying a non-belligerent flag, outside declared war-exclusion zones, that are not carrying contraband or resisting legitimate visit and search by a Persian Gulf belligerent."

An administration official said the policy will put "a little more pressure" on Iran to accept a cease-fire and withdrawal to prewar borders.

Most of Iran's attacks on ships have been in open, territorial waters that will be covered by the new policy, said the official, declining to be identified. By contrast, Iraq's attacks have been confined largely to so-called exclusionary zones that are not covered.

The official noted that the protective coverage does not apply to ships from communist countries and emphasized that skippers must request aid before an American ship renders assistance.

Currently, the Navy has 17 ships in the gulf, including six mine-sweepers, and 10 others in nearby waters. The current protective effort costs $1 million a day.

Carlucci said that he does not anticipate the new policy will require an increase in U.S. forces.

Reagan ordered the new policy during a meeting with national security advisers. The White House had been reluctant to discuss the so-called "rules of engagement" under which the Navy fleet

operates in the gulf.

Along these lines, presidential spokesman Marlin Fitzwater said:

"We don't discuss the specifics of rules of engagement. We don't tell when they're changing. We don't tell when they're new. We do not intend to discuss specific rules of engagement or when specific decisions are made and the reason is obvious. You don't tell the enemy what you're doing."

Reagan's decision followed the return Thursday night of Defense Secretary Frank C. Carlucci and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, from a meeting of North Atlantic Treaty Organization defense ministers in Brussels.

The battles between U.S. and Iranian forces on April 18 came after the Navy attacked Iranian oil platforms in retaliation for the mining of a U.S. frigate.