The Carter administration's new arms control director, retired Lt. Gen. George M. Seignious, came out strongly yesterday for both a new strategic arms limitation treaty (SALT) with the Soviet Union and major increases in United States weapons spending.

In his initial news conference since taking over the job a week ago, the first military man to head the Arms Control and Disarmament Agency said the United States will have to respond with improved weapons to a growing Soviet threat despite the partial limitations in the proposed new SALT II pact.

Seignious did not endorse specific weapons programs or specific budgetary increase, but he came down clearly on the military side of the guns vs. butter debate. "If we are inviting attack by weakness, then all the butter in the world will not provide the sustenance and the happiness for the American people," he said.

"There is a threat level that we have to constantly observe," he added.

Responding to criticism that a career military man would be out of tune with the purposes of reducing arms, the tall, ruddy, retired officer argued that his experience makes him particularly qualified for the job.

"A fellow like me who has been indulging in a profession that selects and distributes and controls and commands arms is a fellow who normally should be qualified to understand which of those arms and what part of the posture of defense can be better, and at what risk, achieved by arms control," he said.

Although a member since last September of the U.S. delegation to the SALT talks, Seignious until a few days ago had given only conditional support to a SALT II pact pending completion of the negotiations. He said yesterday that knowing all the details of recent progress, he can now "drop the qualifier" and say whole-heartedly that SALT II will improve U.S. national security.

If the Soviets are forthcoming at next week's talks between Secretary of State Cyrus R. Vance and Foreign Minister Andrei Gromyko, he said, detail treaty language may be completed by mid or late Januaary and be submitted to the Senate by early Februar.

Noting that the proposed treaty would authorize each superpower to have 2,250 strategic nuclear launchers, a total higher than that in the current arsenal of the United States, Seignious said that perfectly legal Soviet military improvements under SALT II unquestionably will force the United States also to do more.

The new arms control director inidcated that he is particularly concerned about expectable Soviet increases in "the threat to survival of our Minuteman fields," speaking of the basic U.S. intercontinental nuclear missile. He hinted that he would back proposals to build a mightier, more accurate and less vulnerable new missile, which would be permitted under SALT II.

At the same time, he seemed concerned about a possible "shell game" system of deploying a new mobile missile to confuse the Soviets, on grounds that its verification could be a problem.

And he indicated doubts, on the same grounds, about a U.S. request to deploy cruise missiles with monnuour Minuteman fields," speaking of clear warheads.

Seignious, who served in uniform in the early 1970s as chief of the Pentagon's overseas military sales program, declined to express a view about the advisability of continuing President Carter's ceiling on weapons sales abroad.

He did say that "unilateral denial of military equipnment by the United States will not be a successful policy." He described the current talks with the Soviet Union on control of conventional arms as "a first step" but added that even the two superpowers could not by themselves curb the buildup of conventional weapons by other nations.

Seignious said he expects to be closely examined by the Senate Foreign Relations and Armed Services committees when his nomination is before the Senate early next year.

Currently serving under a recess appointment, he gave no details of his plans for the arms control agency except to say that he will attempt "to integrate and influence national policy with a viewpoint on arms control that will enhance our national security programs." CAPTION: Picture, Arms control chief Seignious: "There is a threat level. . ." By Frank Johnston-The Washington Post