More than 50 digital billboards across Minnesota will aim to counter a surge in human-trafficking cases this year, part of a campaign to provide victims with resources to escape what officials on Thursday called "modern-day slavery."

The National Human Trafficking Resource Center has reported that Minnesota has seen nearly 40 cases of human trafficking this year, representing a 12 percent increase over 2015.

The billboards were launched Thursday and will run for three weeks. The campaign is led by anti-human trafficking nonprofit Polaris and Clear Channel Outdoor Americas, and it follows the expansion of "safe harbor" laws in 2015 by Congress. The law encourages states to treat trafficked minors as victims, not as criminals.

"The sad truth, trafficking is a gigantic problem in Minnesota," said U.S. Rep. Erik Paulsen, a Republican who joined Democratic U.S. Sen. Amy Klobuchar in sponsoring the federal legislation. "But this billboard campaign will make a big difference in helping victims reach out to a hot line."

Law enforcement officials, who were on hand Thursday at the State Fair for a news conference announcing the campaign, worry that the 2018 Super Bowl will bring a spike in the number of sex-trafficking victims in the Twin Cities. County prosecutors and law enforcement agencies predict that hundreds of women and girls will be sold on the sex market during Super Bowl, mostly at organized parties and through provocative online ads and social media connections.

Ramsey County District Attorney John Choi said Minnesota has seen a growing number of sex-trafficking cases in recent years, adding that raising awareness of the problem is a key, first step in combating it.

"At one point in time, there were only about 10 of these cases throughout the state of Minnesota. Today we have about 70 or 80 of these cases," Choi said.

The top cities in Minnesota with reported cases include Minneapolis, St. Paul, Rochester, St. Cloud, Blaine and Moorhead, according to the center. Earlier this summer, a Woodbury woman was charged with five felonies for allegedly beating, starving and forcing a servant she brought from China to work for 18 hours a day for almost no pay.

Officials on Thursday said that victims of trafficking are usually sold for sex, while others are forced to work for little to no pay. Recognizing victims of trafficking can be difficult, but some red flags include people who are not free to come and go as they wish, who appear malnourished or physically abused, who do not possess identification documents or who have others speaking for them.

The new campaign makes the 20th such effort, and officials said it has been effective. The hot line number is 1-888-373-7888.

Ricardo Lopez • 651-925-5044