Businesses have added 4.5 million workers under a new program that provides tax breaks for hiring unemployed workers, the Treasury Department said Monday.

It is unclear, however, how many of those workers would have been added without the tax break.

President Barack Obama signed a law in March that exempts businesses hiring people who have been unemployed for at least 60 days from paying the 6.2 percent Social Security payroll tax through December. Employers get an additional $1,000 credit if new workers stay on the job a full year.

Treasury released a report Monday estimating that from February to May, businesses added 4.5 million workers who qualify for the tax breaks. Those businesses are projected to save $8.5 billion in taxes.