NEW YORK - A monthly gauge of online labor demand in the United States dipped slightly in September, as employers remained wary despite signs of economic recovery, a private research group said on Thursday.

Monster Worldwide Inc, an online careers and recruiting firm, said its employment index edged down 2 points to 119 September from 121 in August. The current month's reading is now 25 percent below the 118 mark a year ago.

The September index rose largely due to hiring within the construction and manufacturing sectors, Monster said. Boston, Massachusetts showed the strongest jobs growth of all metropolitan areas.

Despite recent improvements in economic sentiment, U.S. employers continue to exhibit caution when it comes to hiring, said Jesse Harriott, a vice president at Monster Worldwide.

On the upside, demand for workers is firming in the blue-collar segment, with welcome signs of revived activity in construction and manufacturing.

The Monster report comes ahead of the government's monthly employment report to be released on Friday. This is expected to show that while employers shed fewer jobs in September than in August, the unemployment rate will break above August's 26-year high, according to economists polled by Reuters.

In Monster's September report, online job demand rose in just four of the 20 sectors the company tracks, and also rose in four of its 23 monitored occupational categories over the last month.

Job availability decreased in eight of the nine U.S. census regions, with the East North Central as the exception.

The Monster Employment index is a monthly analysis based on a selection of corporate career sites and job boards. The margin of error is approximately plus or minus 1 percent. (Reporting by Camille Drummond, Editing by Chizu Nomiyama)