The Senate Armed Services Committee yesterday approved a bill 12 to 5 that would require President Carter to start registering men 18 through 26 years old next January for possible military service.

The committee's action represented the biggest step the Senate has taken toward returning to registration since the end of the Vietnam war, Draft calls themselves were suspended in 1973.

The bill approved yesterday would not mean the resumption of draft calls, only visits by youths to local draft boards to give the government names and addresses. The bill would suspend the processing of registrants until January 1981.

In the one year interval between next January and January 1981, the president would review the Selective Service process and make recommendations to the Congress. Mental and physical examinations of registrants would not begin until after the report.

A coalition of national organizations is fighting a return to registration, contending it is the first step toward the return of military conscription.

Administration officials acknowledge such opposition is one reason that President Carter and Pentagon leaders have shied away from calling for a return to registration.

The latest expression of the administration was a letter released yesterday from Defense Secretary Harold Brown to Sen. William S. Cohen (R-Maine), an opponent of peacetime registration.

"We are concerned," Brown wrote, that the Selective Service System with its present limited resources might not be able to find and classify draftees in time to get them to training centers within 30 days after the nation mobilized in response to a national emergency.

"However," Brown added, "that circumstance does not lead to the conclusion that peacetime registration is necessary."

Instead, Brown recommended, Congress should approve the extra money the Carter administration is asking for the Selective Service System in fiscal 1980 to finance a network of computers and other streamlining to enable the agency to compile a list of draft age persons without resorting to reimposing the registration requirement.

The Senate Armed Services Committee rejected Brown's advice 12 to 5 and approved a rewrite of bills that call for resuming registration next January. A bill approved by the House Armed Services Committee but still on its way to the floor calls for registering 18-year-old males only, starting in January 1981 - after the 1980 election.

As matters stand, the Senate is likely to vote on registration before the House. One possibility is that an amendment might be offered to the fiscal 1980 Pentagon money bill now on the Senate floor to require registration for the draft.

Chairman John Stennis (D-Miss.) and Sen. Sam Nunn (D-Ga.) of the Senate Armed Services Committee have been at the forefront of the drive to require to return to registration.

Brown, while opposing registration, has said that if it is resumed under Congressional order the requirement should cover women as well as men.

Here is the 12-to-5 breakdown in the yesterday's vote:

For, Stennis, Henry M. Jackson (D-Wash.), Howard W. Cannon (D-Nev.), Harry F. Byrd Jr. (Ind.-Va.), Nunn, Robert Morgan (D-N.C.), John Tower (R-Tex.) Strom Thurmond (R-S.C.), Barry Goldwater (R-Ariz.), John W. Warner (R-Va.), Gordon J. Humphrey (R-N.H.), Roger W. Jepsen (R-Iowa).

Against, John C. Culver (D-Iowa), Gary Hart (D-Colo.), J. James Exon (D-Neb.), Carl Levin (D-Mich.), and Cohen.