Secretary of State Alexander M. Haig Jr. took the U.S. case against Libya to the European allies here today, but said that most of them are likely to make "no change whatsoever" in their own policies toward the Tripoli regime.

Haig, in a brief press conference following the announcement in Washington of steps designed to bring about the withdrawal of Americans from Libya, made a point of saying that he had not asked the allies to take parallel action.

It seemed clear from the substance of remarks which leaked out of a "super-restricted" NATO session and the statements of European diplomats that there is little disposition at this point to follow Washington's lead.

Haig told the highly restricted NATO meeting, according to a participant, that the United States takes "very seriously" the reports of a Libyan terrorist squad aimed at U.S. leaders, whether it is intended for an actual assassination or as a threat. There was no report that the secretary of state unveiled the alleged plot in detail.

Foreign Minister Emilio Colombo of Italy, the only leader who responded immediately to Haig, is reported to have said that "at this moment it is not useful to close any channel of communication" with Libya, despite the fact that a dialogue with that country is "not easy."

Colombo is reported to have indicated that other European nations feel the same way about keeping open the lines of commnications with the Tripoli regime of Col. Muammar Qaddafi. According to one account, European foreign ministers meeting informally in Britain this September decided against reducing or eliminating their diplomatic representation in Libya, as Washington has done, or taking other steps to cut down the flow of people or communication.

The restricted NATO session took place several hours before the Washington announcement. While calling for a "policy of firmness," according to participants, Haig did not disclose the forthcoming U.S. actions at that point.

In his news conference later, Haig said allies and friendly governments had been informed of the U.S. decision. While not asking for parallel steps, he said, "we do welcome their understanding and support.

Asked if he had encountered any resistance to the strategy for dealing with Libya, Haig responded, "I think it's clear some of our European partners will pursue, as they have in the past, their own independent policies with respect to Libya and that would mean in the cases of some, probably most, no change whatsoever."

Apparently in response to European and other concerns, Haig volunteered, "I want to emphasize that the United States, in taking this step, is not engaged in victimizing Libya." He called the Washington actions "prudent steps" for the protection of American citizens in the light of accumulating evidence of Libyan terrorist threats. Nevertheless, he would not rule out the possibility that this is only the first step in a new Libya policy.

Haig denied a report circulating among members of the British press that the United States had discussed the possibility of stronger sanctions with key European allies several days ago but had been rebuffed.

A U.S. official traveling with Haig said there was a sharp contrast between the unilateral American actions announced today regarding Libya and the Carter administration's efforts to arrange concerted economic actions by all the allies against Iran in the hostage episode and the Soviet Union following its invasion of Afghanistan.

The official said none of the allied foreign ministers who met here with Haig had been asked to take steps of their own because of the alleged "hit squads," even in Haig's intimate bilateral conversations.

The Italians, more than probably any other ally, have extensive historical ties with the former Italian colony and maintain extensive commercial ties today.

The French, who have been at odds with Libya on several occasions in recent years, are understood to feel that Libya deserved encouragement at the present time for withdrawing its military forces from Chad. A senior French diplomat reportedly told French reporters here today that Libya seems to be backing away from destabilizing activities in Chad at present.

The French official, asked if Haig had convinced his allies by his remarks at today's NATO meeting, responded sardonically that Haig had "convinced himself."