Washington - Legislation sponsored by U.S. Sens. Richard Blumenthal, D-Conn., and Rob Portman, R-Ohio, calling for an end to human trafficking by government contractors, has been attached to a bill authorizing Defense Department programs for the coming fiscal year.

"Modern-day slavery by government contractors - unknowingly funded by American taxpayers - is unconscionable and intolerable," said Blumenthal, who co-chairs with Portman the bipartisan Senate Caucus to End Human Trafficking.

The Senate voted late Thursday to add the End Human Trafficking in Government Contracting Act as an amendment to the National Defense Authorization Act of 2013.

The federal government has an official "zero-tolerance" policy on human trafficking, and President Obama signed an executive order in September that prohibits government contractors and subcontractors from engaging in activities tied to trafficking, such as charging employees recruitment fees.

But data shows that human trafficking remains a problem.

The National Human Trafficking Resource Center reported that the volume of calls to its hotline went up by 64 percent between 2010 and 2011. And while human trafficking often occurs outside of America's borders, Blumenthal said that 83 percent of the human trafficking cases dealt with by the Department of Justice between 2008 and 2010 involved U.S. citizens.

Additionally, tens of thousands of foreign citizens work for contractors and subcontractors of the U.S. military in Iraq and Afghanistan.

"Current law prohibiting human trafficking is insufficient and ineffective, failing to prevent or punish abuses," declared Blumenthal in a statement issued Friday. "By increasing preventative scrutiny, investigation, and prosecution, this legislation will stop egregious human rights abuses on U.S. military bases, increasing security for our troops, and preventing waste of taxpayer dollars."

The legislation would attempt to improve accountability by requiring contractors to report trafficking-related misbehavior by subcontractors to the inspector general of the agency retaining he contractor's services. The inspector general could then investigate the issue.

It also expands the ability to punish contractors for fraudulent labor practices.

Retiring U.S. Sen. Joe Lieberman, D-Conn., threw his support behind Blumenthal's efforts.

"The United States has long had an official policy of zero tolerance for contractors who engage in human trafficking," Lieberman said.

"This bill will provide better tools for actually enforcing that policy and keeping contract dollars out of the hands of those who engage in horrific abuses of their workers."

In addition to the Blumenthal-Portman measure, Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., has introduced legislation to reauthorize the Trafficking Victims Protection Act, which was first passed in 2000 but which expired a year ago.

Leahy's bill is a broader measure that imposes penalties for human trafficking while providing more funds to state and local governments to combat the problem. While Blumenthal said earlier this month that he supports Leahy's bill, he questioned whether it could pass before the current lame duck session of Congress adjourns at the end of this month.

The Polaris Project, an organization seeking an end to human trafficking, estimates 27 million people, most of them young, are captives as a result of human trafficking. About 17,000 of them cross the border into the U.S. each year, according to the organization.