A last-minute move by the U.S. Senate to renew a controversial wiretapping law, days before it was set to expire on Dec. 31, has dismayed privacy advocates, rights groups and lawmakers who have long opposed the measure.

The FISA Amendments Act of 2008 is an anti-terror measure that authorizes U.S. intelligence agencies to conduct surveillance, including electronic wiretapping, of foreign nationals inside the U.S and overseas who are believed to pose a security risk to the country. Proponents of the measure have described it as a vital tool in the fight against terrorism.

But many others believe that the manner in which the law has been written allows U.S spy agencies to intercept, without a warrant, phone calls and other electronic communications that may involve American citizens as well.

They have decried what they claim is an absolute lack of transparency into how the law is being applied. They have demand more information on how FISA is being interpreted by the non-public court that was created under the law to handle search warrant requests involving foreign suspects on U.S. soil.

The law, which basically amends the Foreign Intelligence Surveillance Act (FISA) of 1978, was scheduled to expire on Dec. 31. But it was renewed in the U.S. Senate by a vote of 72-23 on Dec. 28 and signed into law by President Barack Obama two days later. The U.S House of Representatives voted last September to reauthorize the bill for another five years.

Mark Rumold, a staff attorney at the Electronic Frontier Foundation (EFF), said the renewal of the bill, without any of the several amendments that had been proposed by lawmakers such as Senators Ron Wyden (D-Ore.), Jeff Merkley (D-Ore.) and Rand Paul (R-KY), was disappointing.

It basically gives the government wide leeway to intercept all sorts of communications -- including those involving American citizens -- without needing to establish a particularly strong connection to terrorism, he said.

"My main concern with the law is that we don't know how it is being used," he said.

In fact, even many of the lawmakers who voted to extend the FISA amendments know very little about how, when, where or why it is being used, he said. Apart from lawmakers in the Senate Intelligence Committee and the Senate Judiciary Committee, very few have been briefed on the law and therefore know little about it, Rumold said.