President Carter agreed last night to meet with a group of 19 senators who earlier indicated to him that their support of the SALT II arms control treaty may hinge on his response to their concerns about the U.S. defense posture.

They listed six points in a letter to Carter bearing today's date and written under the direction of Sen. Sam Nunn (D-Ga.).The White House is anxious to persuade Nunn and several others who signed the letter to vote for SALT II, which is pending in the Senate.

In reply, the president last night signed an identical four-paragraph letter to each of the senators that was noncommittal on the six points that they raised but offered, in effect, to negotiate.

"I . . . share your desire to achieve a bipartisan consensus on these issues of long-range national security, strategy and arms control, and to that end we should begin these meetings at an early date," Carter wrote. "I am confident we can find the common ground on which the prompt ratification of this treaty, so important to our national security and the peace of the world, will be achieved."

White House officials said last night that the meetings that Carter proposed had not been set up, but they are expected to be soon.

The signers of the Senate letter represent a potentially decisive voting bloc, and in his reply the president served notice that he is willing to consider further concessions to them in order to win approval of SALT.

Carter has already gone to some lengths to win over Nunn and other defense-minded senators by approving a substantial increase in defense spending for next year and promising a steady rise in military expenditures in the years ahead.

In discussing SALT II, the 19 senators said that they were "concerned" that the terms in the protocol might set a precedent for future arms control pacts. They also indicated they were troubled by the latitude the treaty would give the Soviets in deploying heavy missiles, by the Soviet Backfire bomber threat to the United States, and by limitations on the ways the United States could deploy its new MX blockbuster intercontinental missile.

Turning to general concerns about the military balance between the United States and the Soviet Union, the senators mentioned "ongoing slippage" on the American side, Soviet improvements in tactical aircraft, and a growing Soviet navy that threatens "western supremacy on the high seas."

The senators complained that those and other advances in Soviet weaponry occurred at the same time one arms control treaty was in force and its successor was in negotiation. "The hopes for significant arms control did influence our force planning" and thus may have delayed counters to the "mounting Soviet threat," they wrote.

In light of those developments and their impact on the future arms balance, the senators said that they needed presidential assurances and explanations in these areas:

Plans for narrowing the "window of vulnerability" in the early 1980s when the Soviets will be able to knock out a sizable portion of the U.S. land-based nuclear offense.

Efforts to improve enlistment of high quality people for the armed services.

Administration programs for improving intelligence gathering, including the reconstitution of "our sensitive operational intelligence capabilities."

Administration plans, with date specified, for deploying ground-launched cruise missiles behind the North Atlantic Treaty Organization line in Europe.

"Administration plans to deter and counter" the "aggressive activities in the Third World" by the Soviet Union and its proxies. The senators said such behavior is "inconsistent with the underlying spirit of the SALT process."

Explanations of how SALT II would affect "the attainment of deep cuts" in strategic arms on both sides under a third treaty. The senators added that the administration's assessment of the impact of SALT II should also go into the effect on reducing U.S. and Soviet troops stationed on either side of the NATO line in Europe.

After listing their reservations about SALT II and the general trends of the arms race, the senators concluded: "Because of our concerns, largely covered by this letter, we are uncommitted as to how we will cast our votes on the SALT II treaty and proposed changes."

Sen. Alan Cranston (D-Calif.), one of the administration's chief head counters in its bid to secure approval of SALT, said he viewed the letter "as a favorable development. Most of the very legitimate concerns raised by the senators do not relate directly to the text of the SALT II treat. It's my impression that they hope that those that do can be resolved without killer amendments."

Besides Nunn, the letter was signed by:

Henry L. Bellmon (R-Okla.), Lloyd M. Bentsen (D-Tex.), David L. Boren (D-Okla.), Rudy Boschwitz (R-Minn.), Lawton Chiles (D-Fla.), John Danforth (R-Mo.), Dennis DeConcini (D-Ariz.), Pete V. Domenici (R-N.M.), David Durenberger (R-Minn.), J. James Exon (D-Neb.), S. I. Hayakawa (R-Calif.), H. John Heinz III (R-Pa.), Larry Pressler (R-S.D.), Harrison Schmitt (R-N.M.), Alan K. Simpson (R-Wyo.), Richard Stone (D-Fla.), John W. Warner (R-Va.) and Edward Zorinsky (D-Neb.).