The Reagan administration's cutbacks in federal jobs will likely result in layoffs for nearly 15,000 nonmilitary government workers, including about 4,455 in the Washington area, a congressional task force estimated yesterday.

Previous estimates of job losses had run as high as 12,000 here and 100,000 nationwide over the next 16 months, but members of the task force and officials at various agencies now expect the overall impact to be far less severe.

The latest figures were immediately disputed as far too low by the American Federation of Government Employees (AFGE), the union representing the largest number of federal workers. But task force members said the data is drawn directly from current agency estimates.

"This is our best guess today," said Rep. Michael D. Barnes (D-Md), chairman of the Federal Government Service Task Force. The congressional panel conducted an agency-by-agency survey to determine which ones anticipate making "reductions-in-force" (RIF), a term used when federal employes face layoffs, demotions or reassignments because of government reorganization.

While expressing relief that eventual job losses in the region will amount to less than 1 percent of the total 2.1 million federal work force, Barnes and task force Cochairman Rep. Stanford E. Parris (R-Va), complained the Washington area appears to be suffering a "disproportionate" share of the anticipated layoffs. Federal workers here account for more than 17 percent of the national federal labor force but are expected to absorb 30 percent of the layoffs.

"The overall job losses represent a very small percentage of the federal work force, but if you're one of the people affected, it's significant," said Barnes.

Parris, who had earlier tried to reassure his constituents about the impact of Reagan's budget cuts, said the task force figures were consistent with his initial assessment.

"The estimates have gone from 100,000, and then they were scaled down to 30,000 or 40,000 and now it's about 4,400 -- but we'd rather it could be zero," said Parris.

About 2,000 RIF notices have already been sent to workers around the country, according to the Office of Personnel Management (OPM), and Barnes said more notices will go out in late July and early August. The total number of notices -- which do not automatically result in job losses -- is expected to be about 18,000, including about 5,700 in the Washington area.

Claudia Cooley, chairman of OPM's own task force on personnel reductions, said Barnes' estimates "are more in the ball park of what we're expecting." But she cautioned against trying to come up with hard figures before all the agencies have decided whether they need to reduce their personnel."

Most of the agencies, Cooley said, are hoping to avoid layoffs, relying on attrition, agency transfers and existing vacancies to help them meet congressionally mandated personnel ceilings. OPM itself, she noted, had only determined two days ago that it would not have to resort to layoffs.

The congressional task force received replies from 49 government agencies, 23 of which indicated they would be sending out RIF or change-of-job-status notices to their employes. Angencies reporting the highest number of expected layoffs, particularly in the Washington region, were the Departments of Health and Human Services (8,650 nationally, 2,580 in the Washington area); Commerce (1,400 nationally, 500 locally); Energy (350 and275); Justice (460 and 200); and Transportation (1,250 and 275). Another 26 agencies reported they did not expect to seek any layoffs.

In challenging the task force estimates on layoffs, AFGE President Kenneth Blaylock said, "The administration is playing down the actual numbers that will be affected." Blaylock, whose union represents about 600,000 federal workers, said "We think the impact by the end of this calendar year will be more like 35,000 jobs."