A senior member of the House Armed Services Committee and the head of the National Guard Bureau moved yesterday to forbid National Guardsmen from embarking on free-lance military missions such as the one in which two Americans were killed in Nicaragua Sept. 1.

"If Guardsmen are hired for covert activities," said Rep. G.V. (Sonny) Montgomery of Mississippi, a senior Democrat on the House Armed Services Committee, "you are, in effect, putting active military personnel in a covert operation," because the Guard has become such an integral part of the active forces.

Montgomery, expanding on a statement made on the House floor, said in an interview that he will attempt to prevent Guardsmen, who are part-time soldiers, from engaging in military activities on their own in foreign countries. He said he will introduce legislation to this effect and also will seek to attach such a prohibition to the defense authorization bill now before a Senate-House conference committee.

"I just talked to Gen. Walker and he totally agrees with me that this is not our mission," said Montgomery, referring to Lt. Gen. Emmett H. Walker Jr., chief of the National Guard Bureau, the Pentagon agency that oversees National Guard organizations in the states. A spokesman for Walker quoted the general as saying "there may be a need for such legislation" but that "it may be very hard to enforce."

Dana H. Parker, who was killed assisting the anti-government guerrillas in Nicaragua Sept. 1, was a captain in the Special Forces group of the Alabama National Guard. Two other Americans who were helping the guerrillas at the same time are present or former Guardsmen in the same unit.

One of them, William P. Courtney, is the full-time civilian supply officer for the Special Forces unit as well as a chief warrant officer in it.

"This is just not our role," said Montgomery, who is a major general in the Mississippi National Guard and one of the most influential members of Congress on National Guard matters.

"If they want to do that soldier of fortune stuff, let them leave the Guard," Montgomery said.

He conceded that he was getting into a gray area in seeking to forbid part-time soldiers from going into combat areas on their own time as civilians.

A National Guard Bureau spokesman said the Guardsmen who went to Nicaragua were not under the bureau's sponsorship or in the employ of any government agency, including the CIA or other intelligence agencies.

A three-day audit of Alabama National Guard equipment was conducted following the publicity about the Guardsmen's activity with the guerrillas, the spokesman said, and all major equipment was accounted for.

Any supply of National Guard equipment to Central America, said National Guard Bureau spokesman Dan Donohue, would have been "patently outside the bounds of any authority" from the state and federal governments.

The National Guard has only two Special Forces groups, in Alabama and Utah. They were formed in 1961 and have an authorized strength of about 650 persons each, not counting headquarters and support personnel.

In a related development, the Reagan administration said it made a policy decision "several months ago" against soliciting private funds or third-country support for anti-government guerrillas in Nicaragua to make up for CIA aid being terminated by Congress. The administration added, though, that it has "not discouraged" such alternative funding.

State Department spokesman John Hughes issued the statement after reports in several newspapers last weekend that the insurgents raised millions of dollars from foreign governments and U.S. private citizens after Congress voted an end to undercover CIA funding.

"There are numerous private U.S. organizations that make no secret of providing assistance to groups overseas," said Hughes in reference to aid to the anti-Sandinista guerrilla groups known as contras. "Provided U.S. funds are not used, we do not discourage other countries from providing support. Nor have we discouraged private U.S. contributions which, as I understand it, are entirely legal."

Hughes did not explain how that interpretation squares with the Neutrality Act, which makes it a crime under U.S. law to "furnish the money for" a military or naval expedition against a state with which the United States is not at war.

The spokesman said both the Department of Justice and the Customs Service are "looking into" the case of Civilian-Military Assistance, the Alabama organization that supplied equipment to the rebels and arranged for Americans to assist the contras in camps along the Nicaraguan-Honduran border. Two of the Americans, Parker and James Powell III, were killed when a rebel helicopter was downed inside Nicaragua.

On Capitol Hill, members of the Senate Select Committee on Intelligence said CIA Director William J. Casey and other officials are scheduled to brief them today about private aid to the contras and related topics. Members of the House Permanent Select Committee on Intelligence said they expect a similar briefing later this week.

Several lawmakers said they do not expect the administration to make a major effort to win new funds for the anti-Sandinista forces during the current short session of Congress. An administration official agreed that "no one really sees any prospects for getting any funds through the normal legislative process at this time."

The official said, however, that the administration will wage a hard fight against restrictions, such as the House has imposed in the past, forbidding spending from contingency funds and other special accounts to aid the contras.