The nation's unemployment rate hit its lowest level of the decade last month, dropping to 6.0 percent, the Bureau of Labor Statistics reported yesterday.

The decline from June's 6.1 percent rate was small, but it was another indication that the economy is growing faster than expected. Some experts had seen the rate's decline in June as a statistical aberration, but the July rate -- achieved through substantial employment growth and a decline in the number of jobless Americans -- did not appear to be illusory.

"The unemployment report was really a super report on the labor market," said Allen Sinai, chief economist for Shearson Lehman Bros. Inc. "The incredible job-creating machine in the United States is now creating jobs in services and goods alike, suggesting the economy has a much stronger underpinning than might have been suspected. The report is a very positive sign for continued growth in income, consumer spending and a very solid rise in real GNP {gross national product} for the second half."

Sinai and other economists cautioned that stronger growth brings with it an increased danger of inflation, a possibility underscored Thursday when the Reagan administration raised its forecast for price increases in 1987 by a full percentage point, to 4.8 percent. Although most of the increase was attributed to such one-time factors as higher oil prices and the weaker dollar, inflation could pick up over the long term if those influences lead to demands for higher wages.

Policymakers at the Federal Reserve, which governs the growth of money and credit, "must be genuinely concerned about a resurgence of wage increases toward the end of this year and the early part of next," said economic forecaster Joel Popkin. "The Fed is not going to be able to let interest rates come down over the next six months."

President Reagan hailed the new unemployment figures in an appearance before White House reporters, calling the rate "remarkable news."

"This breakthrough ... does not occur in a hyperinflated economy as it did in 1979 {when the rate last hit 6.0 percent} but is based instead on strong growth and steady job creation," Reagan said.

Labor Secretary William Brock used the drop in unemployment to press Congress to modify the trade legislation now before a House-Senate conference committee. Reagan has threatened to veto the trade bill as protectionist if it is not altered.

"We should all keep in mind that the true test of an effective trade bill will be one that enables the U.S. to maintain a productive, competitive economy that continues to generate expanded employment opportunities for all Americans," Brock said in a statement.

The July figures showed an increase of 470,000 in the number of employed workers, to 112.7 million, and a decline in the number of jobless people unsuccessfully seeking work -- the official definition of unemployment -- of 36,000 to 7.2 million, the Bureau of Labor Statistics said.

Altogether, the ranks of the employed have grown by 1.7 million since the beginning of 1987 and nearly 3 million in the last 12 months. In July, the number of jobs increased in both the manufacturing and service sectors. Manufacturing gained 70,000 jobs -- despite a loss of 40,000 jobs in auto production -- and service employment rose by 80,000, according to the BLS survey of households. A separate survey of business found that employment increased 304,000 last month, including 74,000 new jobs in manufacturing.

The unemployment rate for July, which includes all civilian workers, was one percentage point lower than it was 12 months ago, and more than half a point below the 6.7 percent rate registered at the beginning of this year. An unemployment rate that includes members of the armed forces also fell in July, to 5.9 percent from 6.0 percent the previous month. All unemployment figures are adjusted for seasonal factors.

The last time the civilian unemployment rate was 6.0 percent was in December 1979. Joblessness climbed steadily after that, peaking at 10.8 percent in November and December 1982 during the dark days of the recession, hovering at 9.6 percent in 1983, then falling rapidly in 1984 and averaging 7.2 percent in 1985 and 7.0 percent in 1986.

Most economists say unemployment will decline more slowly during the rest of 1987 than it has so far this year. Sinai said he expects the rate to fall to 5.5 percent by December and to average 6.1 percent for the year. Thursday, the Reagan administration issued a revised economic forecast predicting an average unemployment rate for 1987 of 6.2 percent, one-half point below its January forecast.

Popkin contended, however, that the unemployment picture is not quite as rosy as the raw numbers would indicate. He noted that there remains a substantial pool of skilled workers who have been unemployed for long periods of time. In the second quarter of 1987, 1.7 percent of the labor force had been unemployed for 15 weeks or longer, only slightly less than the 1.9 percent in that category during the same period in 1986.

Testifying before the congressional Joint Economic Committee yesterday, Bureau of Labor Statistics Commissioner Janet Norwood noted another disturbing aspect of the July figures: The number of workers employed part-time because they cannot find full-time work increased by 325,000 in July to 5.5 million, or 4.8 percent of those employed.

In May 1979, by contrast, those working part-time for economic reasons made up 3.3 percent of the total employed, even though the unemployment rate was 5.8 percent, close to what it is now.

Norwood also noted that jobless figures tend to fluctuate more in the summer, when an influx of college students sometimes throws off the adjustments the bureau makes for seasonal factors. However, she pointed out, total employment has risen by 900,000 since April, an increase too large to be caused solely by statistical variation