Ohio Senator Rob Portman's bill aims to curb human trafficking by holding websites accountable for illegal activity, but critics say it would undermine a core principle that has allowed the internet to flourish

The tech sector is digging in for battle with US lawmakers over a proposed law aimed at curbing human trafficking by holding website owners liable for illegal content posted by others.

Backers of the bill argue it would give law enforcement more tools to crack down on websites promoting sex trafficking, but internet firms and civil liberties advocates argue the measure would undermine a core principle of internet freedom.

The bill introduced by Senator Rob Portman with bipartisan backing appears aimed at the website Backpage which has been accused of facilitating child prostitution and sex trafficking.

"The internet has revolutionized illegal sex trafficking, and federal law simply has not kept pace," Portman said in introducing the legislation.

But critics say the bill aimed at modifying the liability provisions of the Communications Decency Act could eliminate the free-speech underpinnings of the internet, giving websites immunity from content posted by others.

"The proposed legislation would have a devastating impact on legitimate online services without having a meaningful impact on ending trafficking crimes," said a letter to lawmakers from the Computer and Communications Industry Association and other groups which include tech giants such as Google, Facebook, Microsoft.

Gary Shapiro, president of the Consumer Technology Association, representing some 2,000 tech firms, said existing law "is the legal underpinning for America's world-leading internet industry" that has allowed the online sector to flourish.

"By clarifying that online platforms are not liable for the posts of their users, Congress enabled internet platforms ranging from major companies to startups to host reviews, pictures, status updates and other user content," Shapiro said in a statement.

Without this immunity, he said, "internet platforms would be forced to censor content heavily and faced with crushing legal liability."

Eric Goldman, who heads the Santa Clara University High Tech Law Institute, said the bill "potentially implicates every online service that deals with user-generated content, which would make this an unusually wide-ranging bill."

Other critics say legal tools are available to crack down on websites that promote human trafficking without affecting free speech rights.

"Internet companies agree that further steps must be taken to end human trafficking," said Michael Beckerman of the Internet Association, which includes Amazon, Facebook, Google and other online operators opposing the legislation.

"The internet industry is committed to working with Congress to... provide public and private partners the resources necessary to combat human trafficking."