WASHINGTON (AP) _ Americans' wages, salaries and benefits rose 4.6 percent over the past 12 months, much less than the previous year, the government said today.

The Labor Department said the employment cost index, considered one of the best gauges of inflationary wage pressures, rose 4.6 percent for the year ending in March 1991. A year ago, wages and benefit costs had soared 5.5 percent for a similar 12-month period.

The government noted that the increase for the past 12 months was below the 4.9 percent increase for the year ending in December. It ascribed the more moderate increase to a marked slowdown in benefit costs.

Analysts said the employment cost report showed that wage inflation has abated slightly and may have been a factor in the Federal Reserve's move this morning cutting the discount rate by one-half point to 5.5 percent.

''I think it was a supporting piece of evidence,'' said Robert Brusca, an economist with Nikko Securities Co.

Also today, a separate Labor Department report showed that workers covered by collective bargaining reports settled so far this year won annual wage gains averaging 3.4 percent over the life of the pact, an improvement over the 2 percent annual gains of the settlements the new contracts replaced.

While moderate increases in compensation costs are good for U.S. businesses, they often hold workers' paychecks from keeping pace with inflation.

Inflation shot up 6.1 percent last year, well over the 4.9 percent increases workers received. So far this year, consumer prices have risen at an annual rate of 2.4 percent. For the January-March period, workers' compensation costs increased 1.2 percent.

Wages and salaries increased 4.2 percent for the year ending in March, compared with 4.3 percent for the 12-month period ending in December. For the January-March period, wages and salaries rose 1.2 percent.

For the past year, there was a substantial slowdown in benefit cost increases, which rose 5.9 percent for 12 months ending in March, down from the 6.7 percent spurt for the year ending in December.

In the first quarter of 1991, benefit costs increased 1.4 percent, well below the 2 percent increase for the same three-month period a year ago.

The slowdown in benefit costs occurred as relatively large increses in health insurance costs and workers' compensation insurance rates were dampened by lower increases in pension costs and non-production bonuses, the government said.

Compensation increases in private industry for the past year were higher for white-collar workers, who saw their wages, salaries and benefits rise 4.7 percent, than for blue-collar workers, who received 4.3 percent increases.

Among white-collar jobs, increases ranged from 4.2 percent for clerical and sales workers to 5 percent for executives, administrators and managers.

Gains for blue-collar workers varied from 3.1 percent for transportation and material-moving occupations to 4.4 percent for craft workers and for machine operators, assemblers and inspectors.

Over-the-year compensation gains were about the same in private goods- producing industries, which received 4.4 percent, as in service-producing industries, where costs rose 4.5 percent.

Within service industries, gains ranged from 2.3 percent in transportation to 6.5 percent in hospitals and business services. Gains among goods-producing industries ranged from 4.2 percent in construction to 4.5 percent in non- durable goods manufacturing.

Non-union workers received better wage gains than their union counterparts, continuing a trend that's been ongoing since 1983, the Labor Department said. Non-union workers received compensation gains of 4.5 percent, compared to the 4.1 percent gains of union workers.

State and local government workers received compensation gains of 1.6 percent for the January-March period, the Labor Department said.