WASHINGTON, March 31— The United States has begun negotiations with the Pakistani Government for the evacuation of private American citizens and the dependents of American officials from East Pakistan.

State Department officials said today that the “mechanical” aspects of chartering civilian airplanes, gathering the Americans, and arranging for their safe transportation to air ports were now being worked out. They declined to speculate when the flights would begin.

The United States officials avoid the word “evacuation” and refer instead to “thinning out” of the American colony in East Pakistan.

Officials conceded that they were still concerned for the safety of 2,200 American citizens in West and East Pakistan. The previous estimate of 1,100 Americans in East Pakistan has now been lowered to 750; approximately 635 of whom are believed to be in Dacca and 115 in Chittagong.

The latest information avail able here indicates that the Pakistani Army is in full control of Dacca and Chittagong. Disturbances reported earlier in Chittagong have now ended, officials said. However, they conceded, the curfew and lack of communications have afforded virtually no credible in formation about conditions in the countryside.

Well ‐ informed nongovernment sources here charged that the Administration was receiving—but not making public—reports of heavy bloodshed in East Pakistan. There were several reports that at least one official cablegram to the State Department from outside Pakistan had used the expression “selective genocide.”

United States officials tended to resist—without formally denying—report that such a message had been received.