WASHINGTON (Reuters) - Attorneys general from 50 U.S. states and the District of Columbia will announce on Thursday they are joining forces with major telecommunications providers, including AT&T Inc, Verizon Communications Inc, Comcast Corp, Sprint Corp and T-Mobile Us Inc, to crack down on unwanted robocalls, according to a document seen by Reuters.

The state AGs and companies are unveiling non-binding “anti-robocall principles.”

The companies agreed to take steps to reduce unwanted calls and work with state attorney generals over “recognized scams and trends in illegal robocalling” and to update states on “potential additional solutions for combating illegal robocall.”

Robocalls are the single largest source of complaints to the Federal Communications Commission, Federal Trade Commission and to many state attorneys general offices. Congress has been working on legislation to crack down on unwanted calls in recent months.