The government has completed 97 percent of the Year 2000 computer fixes on its most critical electronic systems, the Office of Management and Budget said yesterday in a quarterly report on federal Y2K efforts.

Of 24 large Cabinet departments and agencies that provide vital federal services, 15 have wrapped up their Y2K repairs and tests and nine agencies have a combined total of 217 critical systems that still require repair or replacement. The Defense Department owns 161 of those systems and appears likely to keep working on Y2K right up to Jan. 1.

The government's mobilization to assess and fix its 6,343 "mission critical" systems will cost an estimated $8.34 billion, up $290 million from three months ago, OMB reported.

Two years ago, congressional investigators and some Y2K experts expressed skepticism that the government could fix its systems by Jan. 1, 2000. But virtually all major agencies expect to wrap up work on their internal systems within the next few weeks, the OMB report indicated.

The Federal Aviation Administration has fixed and tested air traffic control systems, and key agencies providing benefits and loans, such as the Social Security Administration, the Education Department and Veterans Affairs Department, have completed work on 100 percent of their systems, OMB said.

"We feel we're making really solid progress as we come into the home stretch," said Linda Ricci, the OMB spokeswoman.

Based on industry reports, the White House expects the nation's electric power and telephone systems will not suffer any widespread Y2K problems. But the White House official in charge of Y2K issues, John A. Koskinen, has suggested that some states and cities may be at risk of Y2K breakdowns if local government repairs are not finished in time.

The Year 2000 problem stems from the use of two-digit date fields in many computers, which may interpret "00" as 1900, not 2000, and shut down or otherwise malfunction. Although the technical fixes for the date change are relatively simple, the widespread scope of the problem has forced a number of agencies to defer other technology projects and focus almost exclusively on Y2K.

OMB, for example, describes the Defense Department as "conducting the largest, most complex testing effort in its history" to detect potential Y2K glitches. The military has tested its logistics supply channels in the U.S. European Command and tested several weapons systems, including rockets, at White Sands, N.M.

Despite the tests, OMB said, "the interconnection of so many complex systems increases the likelihood that DOD will experience some Y2K difficulties."

The military hopes to minimize any Y2K disruptions through emergency backup plans, OMB said. The Pentagon "is conducting tabletop exercises to help prepare DOD leadership for the potential impact of Year 2000 on national security" and the chairman of the Joint Chiefs of Staff will lead "a national-level exercise conducted under scenarios of multiple Y2K failures in order to assess the ability of DOD to respond with timely decisions in a Year 2000-degraded environment," OMB said.

The other worrisome area involves public services administered through state agencies, such as food stamps, child nutrition, Medicaid, heating assistance for low-income people and welfare. Many states will not finish testing data exchanges with their federal partners until November and December, leaving little time to fix any previously undetected glitches.