Universal Military Training graduates may be called into active service for 18 months if the armed forces can't be cut, the Senate Armed Services Committee warned Saturday. The committee also asked the Senate to approve U.M.T. this year, if for only a small-scale start.

Meanwhile, in Washington yesterday, Rep. Shafer (R-Mich.) said a House coalition of Republicans and southern Democrats will buck the Administration again this week by opposing the bill to establish U.M.T.

The measure will reach the House floor Tuesday.

The committee would turn the training program into a school for draftees until circumstances allow a cutback in the armed forces so that the trainees could go into the Reserve without active service.