U.S. construction firms added 45,000 jobs in April and 280,000 over 12 months, as the sector's unemployment rate fell to a nine-year April low of 7.5 percent, according to an analysis released May 8 by the Associated General Contractors of America.

Although, the AGC noted that even as the industry continues to expand, growth around the U.S. has been erratic and inconsistent.

"Construction employment resumed strong growth in April after slipping in March and is now growing at more than double the growth rate for total non-farm employment," said Ken Simonson, the association's chief economist. "Nevertheless, job growth remains spotty with the nonresidential building sector losing jobs even as other construction sectors expanded."

Construction employment totaled 6.38 million in April, compared to 6.33 million in March and 6.10 million in April 2014, Simonson noted.

Residential building and specialty trade contractors added 23,600 jobs (1 percent) since March and 153,300 jobs (6.7 percent) over 12 months.

Nonresidential contractors—building, specialty trade, and heavy and civil engineering construction firms—hired a net of 20,800 workers for the month and 126,100 (3.3 percent) since April 2014.

Simonson added that the number of unemployed construction workers, 652,000, is at the lowest level since 2001. The construction economist cautioned that reports of construction worker shortages are likely to grow over the coming months. "With construction employment likely to continue to expand for the foreseeable future, labor conditions are likely to get even tighter."