WASHINGTON » U.S. employers posted fewer job openings in September after advertising more in August than first estimated. The report suggests hiring will likely remain modest in the coming months.

The Labor Department said today that job openings dropped by 100,000 to 3.56 million, the fewest in five months. August’s openings were revised up to 3.66 million.

The number of available jobs has jumped about 63 percent since July 2009, one month after the recession ended. It remains well below the more than 4 million jobs a month advertised before the recession began in December 2007.

The job market remains very competitive. With 12.1 million people unemployed in September, there were 3.4 unemployed people, on average, competing for each open job. In a healthy economy, that ratio is roughly 2 to 1.

Employers filled fewer available positions in September than in August. And the number of people who quit fell to the lowest level in 10 months. That’s a bad sign for the job market, because it suggests workers see fewer opportunities to move to a better job. Workers tend to quit when they have other job offers.

One positive sign in the report: Layoffs fell.

Hiring looked a little better in October, according to the Labor Department’s employment report released Friday. Employers added 171,000 jobs last month and hiring in August and September was better than first estimated.

The unemployment rate rose to 7.9 percent last month from 7.8 percent in September. But that was because more Americans began searching for work, likely reflecting increasing optimism about their chances.

The employment report measured net hiring and unemployment, while today’s report looked at total hiring, layoffs and quits.

Job openings fell in manufacturing, construction, hotels and restaurants, and in government. There was also a big drop in openings in professional and business services, which includes both high-paying jobs such as architects and engineers as well as temporary services.