The Carter administration launched a concerted campaign yesterday to stress publicly its view that the militants holding the American hostages in Tehran may be Marxist terrorists beyond the control even of revolutionary leader Ayatollah Ruhollah Khomeini.

The administration's move, which was kicked off publicly by White House press secretary Jody Powell and continued last night by President Carter, was described by reliable sources as a deliberate effort to impress on public opinion, both in the United States and abroad, that the hostages' captors should be viewed not as students but as terrorists seeking to create chaos for their own political ends.

"The most powerful single political entity in Iran consists of the international terrorists or the kidnapers who are holding our hostages," the president told about 80 members of Congress last night at a White House briefing on the crises in Iran and Afghanistan.

"Whenever there has been a showdown concerning the hostages between Khomeini or the Revolutionary Council versus the terrorists, the terrorists have always prevailed. This small group of people . . . has achieved with the holding of the American hostages a great and significant political influence in Iran. They don't necessarily have as one of their prime interests the integrity of Iran as a nation or the well-being of the Iranian people or even the security of the country in which they live."

According to the sources, the administration wants to underscore to the American public and to other countries that efforts to negotiate with the nominal authorities in Iran for release of the hostages may be frustrated by the inability of those authorities to control the militants holding the U.S. Embassy compound.

The administration feels it is necessary to make that point clearly, the sources said, partly in hopes that it might spur more moderate Iranians who want a peaceful resolution of the crisis to put pressure on the militants to be more flexible in their demands.

In addition, the sources said, although the administration is still determined to purse every possible peaceful approach, it wants to make clear that the motives of the militants may run counter to a peaceful outcome and ulimately force the United States to consider more drastic action.

A resort to force or other tougher steps is still not under consideration, the sources stressed. But, they added, it it should become necessary, the administration wants to prepare the public in advance to understand that the blame rests with the intransigence of the militants.

The line taken by Powell and other senior administration officials is not new. It has been stated regularly by White House and State Department officials throughout most of the 10-week crisis.

But, the sources said, a top-level White House decision to give it a coordinated new emphasis was made this week following President Carter's meeting with United Nations Secretary General Kurt Waldeim on Sunday, Carter, the sources said, was deeply disturbed and impressed by Waldheim's description of the chaos and breakdown of normal governmental authority that he encountered on his mission to Tehran last week.

At his regular press briefing yesterday, Powell seized on the first question about Iran to say -- reading from prepared notes -- that the United States has "serious questions about exactly who is in charge in Iran."

These questions, he added, "involve the motives of the terrorists who are holding the American hostages. There is some reason to ask whether they may not see chaos and disintegration in Iran as being beneficial to their ends."

"The real question is if they are concerned with the fate of their fellow citizens in Iran, as opposed to their own power and political ends," powell asserted.

He said the composition of the group controlling the embassy may have changed since the compound was seized on Nov. 4, and he questioned whether those now in control can legitimately be called "students," as they claim.

The militants, Powell charged, openly follow "a rather radical and certainly Marxist line" that "is not compatible with the needs of the people of Iran for a stable economy and the right in practice their religious beliefs." Under questioning, he said he wasn't trying to imply that they are under the control of the Soviet Union.

Charging that Iranian authorities have given "little indication" of being able to "exercise the will and resources to control the people at the embassy," Powell said: "It is time for those in Iran who do care about their country . . . to review whether they wish to allow a small group of so-called students to place these large concerns in peril."

Prior to Powell's on-the-record remarks, a senior State Department official, briefing reporters on the understanding that his name not be used, enunciated many of the same points.

That, the official said, is why the administration is determined to press its campaign to have the United Nations impose economic sanctions against Iran, even though some diplomats reportedly have warned that sanctions will make the Iranians even more inflexible about negotiating.

"It's not altogether clear that an order could be given at any level [in Iran] and result in the quick release of the hostages," the official said. "These terrorists . . . are swimming in a sea of support from the Iranian government and people and we would like to separate them from that support."

Reliable sources said instructions to stress this line -- emphasizing that the militants are "terrorists" with "Marxist" attitudes and apparently unresponsive to authority -- were given by Powell yesterday to other government spokesmen and information officers dealing with the Iranian situation.