The Joint Chiefs of Staff endorsed the new strategic arms limitation treaty yesterday as "a modest but useful step" provided it is accompanied by substantial new military spending.

The five chiefs "are not raging enthusiasts for many features of the treaty," as Adm. Thomas B. Hayward put it, but under intense senatorial questioning the chiefs reiterated their overall support for SALT II and their belief that the treaty is "adequately verifiable."

As the Senate Foreign Relations Committee continued hearings on the treaty, John Glenn (D-Ohio) said he thought the chiefs were "damning it by faint praise," but Gen. David C. Jones of the Air Force, chairman of the JCS, said he would not use that phrase.

Moreover, Jones explicitly rejected the idea of reopening substantive negotiations on SALT II with the Soviets, and he disputed the suggestion that the country would be better off by abandoning the SALT process or rejecting this treaty.

Repeating the phrase the chiefs worked out to describe their position, Jones said the treaty was "a modest but useful contribution, but it doesn't solve the whole problem. That's our advice and we hope you will take it."

In a detailed prepared statement and in answers to senators' questions, the five chiefs echoed the message given earlier by both Defense Secretary Harold Brown and Secretary of State Cyrus R. Vance: Even with SALT II, the United States will have to increase its spending on strategic forces by $10 billion or more in the next six years.

But in the special echo chamber that the nation's military leaders create, that message came through yesterday louder and more forcefully than it had in earlier hearings on SALT II in this committee.

Senior officials of the Carter administration expressed satisfaction that the chiefs had given broad support for the treaty while declining to adopt any of the opponents' substantive or tactical positions.

However, skeptics and opponents thought they found some support in the many qualifying clauses that dotted the chiefs' comments.

Sen. Jesse Helms (R-N.C.), for example, pressed Gen. Lew Allen Jr. of the Air Force on qualifications in the chiefs' prepared statement about the verifiability of SALT II - that is, America's ability to monitor Soviet compliance with the treaty.

The statement said that verification "will pose a stern challenge to our varied and highly capable intelligence systems," and that U.S. ability to verify different specific elements of the treaty will vary "substantially."

Under questioning from Helms, Gen. Allen said there were some points that might not be possible to verify with high confidence, but the chiefs assessed the potential significance of cheating in these areas and "our conclusion was that it is all right" - provided, he went on, that the United States aggressively improves intelligence-gathering.

Helms replied that he sympathized with citizens watching the hearings on television who had to decide what the general had just said. "I think you said 'yes and no,'" Helms commented with a grin, and Allen did not dispute him.

The chiefs' testimony illuminated the complex web of interests involved in the SALT process. When the five pillars of braid and brass who lead the country's armed services sat down in a row at the witness table, there wasn't a civilian official in sight - only military aides. This was the military's day, as Jones affirmed under questioning from Frank Church, (D-Idaho), the committee chairman.

Will you "give us your honest advice" even if it differs from the position of the president and secretary of defense, Church asked.

"Yes, sir, we pledge to do so," the general replied.

In their carefully drafted statement, read by Jones, the chiefs declared: "With or without SALT, the United States needs to do far more than we have done in recent years to strengthen and modernize our strategic forces lest the trend toward Soviet superiority become irreversible."

The chiefs said continued cutbacks starting in the Nixon administration "lowered . . . the incentives for the Soviets to negotiate significant reductions in strategic arms."

"The most serious concern of the Joint Chiefs of Staff," their statement said, "is the risk that SALT II could be allowed to become a tranquilizer to the American people," one that would disguise the "urgent need to proceed resolutely and deliberately with a well-thought-out program of force modernization . . ."

The chiefs' statement acknowledged that they had always wanted the Soviet's Backfire bomber to be counted under SALT limits, and that the provision permitting the Soviets to retain 308 "heavy" supermissiles also troubled them. They also said they would have preferred a treaty that called for much deeper cuts in both superpowers' arsenals.

But they added that restraints on the Backfire's rate of production and on the number of warheads the "heavy" missiles can carry (ten) were both significant. And the chiefs enumerated half a dozen provisions of SALT II which they said "operate primarily to our advantage."

These included provisions forcing the Soviets to dismantle 250 strategic systems, limiting the numbers of warheads rockets can carry, limiting the introduction of new rockets, banning the Soviets' SS16 mobile missile, and requiring that the Soviets not interfere with U.S. "national means" of observing their strategic programs.

"On the other hand," the chiefs said, "the specific limits on the United States are quite nominal" and permit all the strategic programs now on Pentagon drawing boards to go ahead.

"The danger to the United States does not arise from any specific limitations in the agreement, but from potential consequences of unilateral actions or inactions in the past - and, if we are not careful - in the future."

If the United States undertakes new military programs - and senior Carter administration officials have been promising them repeatedly all week in these hearings - and if future negotiations lead to substantial reductions, "history will record SALT II as a step forward," the chiefs said.

Without a commitment to both those points, they added, "we will find SALT II made little difference and may have been a net loss."

The chiefs noted that in 1972 their predecessors recommended approval of the SALT agreements provided certain new American programs were undertaken, but that some of those programs (the B1 bomber, for example) were later canceled. Had that advice been heeded the new chiefs said, "we would face less perilous strategic prospects . . ."

Yesterday morning, Defense Secretary Brown and his undersecretary for research and engineering, William J. Perry, testified. Joseph R. Biden Jr. (D-Del.) pursued the issue of the 308 Soviet heavy missiles that has concerned numerous treaty critics. Howard H. Baker Jr. (R-Tenn.), the Senate minority leader, has said these 308 missiles represent "a fatal flaw" in the treaty.

Biden elicited affirmative answers from Brown to a series of questions meant to establish that these 308 missiles do not give the Soviets any great advantage that they couldn't achieve with other rocket systems. At the end of his questioning, Biden said: "I hope this puts an end to what I think is posturing on this issue."

Baker could not reply; he was absent from yesterday's hearing. CAPTION: Picture, GEN. DAVID C. JONES . . . warns of SALT "tranquilizer"