WASHINGTON – At least 3.1 million Americans are employed in green jobs, a sector that now accounts for about 2.4 percent of the nation’s total employment, the Labor Department said Thursday.

The report represents the first time the department’s Bureau of Labor Statistics has come up with an official count of environmentally friendly jobs, an emerging part of the economy and a centerpiece of President Obama’s stimulus plan.

Environmental groups cheered the report as an affirmation that green jobs are a real factor in the nation’s economic growth. Obama has set a goal of creating 5 million green jobs and his stimulus plan provided $80 billion to help boost this sector.

Republicans have questioned Obama’s focus, saying there has been little to show for all the money spent. They point to boondoggles like the administration’s decision to pump $528 million into California solar company Solyndra before it collapsed.

The report makes no assessment of when the jobs were created and says its figures are from 2010. It found 2.3 million green jobs in the private sector and 860,300 in the public sector.

In the private sector, manufacturing had the greatest number of green jobs with about 461,800, about 4 percent of all manufacturing employment. Construction was second, with about 372,000 jobs.

Vermont had the highest proportion of green employment at 4.4 percent, while Florida had the lowest at 1.3 percent. California had the largest number of green jobs, with 338,000 workers.

But the report is sure to add to the debate over how exactly to define green jobs. Some may wonder how the report can say nuclear power plant employees have green jobs, but not workers who make bicycles.

The answer lies in a two-part definition that the BLS developed in 2010. The first part counts “output-based jobs” that produce goods and services benefiting the environment or conserving natural resources. That includes a company producing solar panels or a farmer growing organic tomatoes.

The second part of the definition covers “process-based jobs” where workers make a company more environmentally friendly or use fewer natural resources. That would include an employee at a manufacturing plant in charge of recycling, even if the plant itself is not green.

A nuclear power plant is green because it produces energy without causing harmful greenhouse gases, said Rick Clayton, chief of the division of administrative statistics and labor turnover at BLS.

The report released Thursday includes only jobs under the first part of the definition. A second report counting process-based green jobs will be released later this year.