Internet heavyweights Facebook, Google, Twitter and Yahoo! have joined ranks to oppose legislation in the US Congress intended to crack down on online piracy.

In a joint letter, the firms on Tuesday said they "support the bills' stated goals -- providing additional enforcement tools to combat foreign 'rogue' websites that are dedicated to copyright infringement or counterfeiting."

"Unfortunately, the bills as drafted would expose law-abiding US Internet and technology companies to new uncertain liabilities, private rights of action, and technology mandates that would require monitoring of websites," they said in the letter to the House and Senate judiciary committees.

"We are concerned that these measures pose a serious risk to our industry's continued track record of innovation and job-creation, as well as to our nation's cybersecurity," the Internet giants said.

The separate bills introduced in the House and the Senate would give the US authorities more tools to crack down on "rogue" websites accused of piracy of movies, television shows and music and the sale of counterfeit goods.

The Stop Online Piracy Act has received some bipartisan support in the House of Representatives and is the House version of a bill introduced in the Senate in May known as the Theft of Intellectual Property Act, or Protect IP Act.

The legislation has received the backing of Hollywood, the music industry, the Business Software Alliance, the National Association of Manufacturers, the US Chamber of Commerce and other groups.

But it has come under fire from digital rights and free speech organisations for allegedly paving the way for US law enforcement to unilaterally shut down websites, including foreign sites, without due process.

The Obama administration has come in for some criticism for shutting down dozens of "rogue" websites over the past year as part of a crackdown known as "Operation in Our Sites."

The websites which have been shut down include dozens selling mostly Chinese-made counterfeit goods, including golf clubs, Walt Disney movies, handbags and other items.

In the letter, the Internet companies said "we cannot support these bills as written and ask that you consider more targeted ways to combat foreign 'rogue' websites dedicated to copyright infringement and trademark counterfeiting."

House Judiciary Committee chairman Lamar Smith, a Republican from Texas, has said the bill "helps stop the flow of revenue to rogue websites and ensures that the profits from American innovations go to American innovators.

In addition to Facebook, Google, Twitter and Yahoo!, the other companies signing the letter were AOL, eBay, LinkedIn, Mozilla and Zynga.