Washington (AFP) - The US Justice and Intelligence chiefs on Monday formally asked Congress to renew a crucial surveillance law, setting up a battle with civil libertarians over collection of Americans' personal data.

Director of National Intelligence Dan Coats and Attorney General Jeff Sessions are seeking a reauthorization of Title VII of the Foreign Intelligence Surveillance Act (FISA), whose key Section 702 allows the National Security Agency to tap the communications of foreigners located abroad for intelligence purposes.

That power has become crucial, intelligence officials say, in preventing terror attacks and other major threats.

But in that process, critics say, the NSA also scoops up the communications of Americans, violating their privacy rights and leaving the information freely available to other intelligence agencies and the Federal Bureau of Investigation.

Section 702 was passed in 2008 to replace a previously secret and illegal warrantless wiretap program instituted after the September 11, 2001 attacks.

It gave intelligence agencies the power to tap the phone calls and emails of foreigners located outside the United States carried by US communications companies.

It was renewed in 2012 for five more years, and comes up for reauthorization again by December 31.

Section 702 "produces significant foreign intelligence that is vital to protect the nation against international terrorism and other threats," Coats and Sessions said in a letter to congressional leaders.

They asked for unchanged and permanent reauthorization, arguing that the statute "provides a comprehensive regime of oversight by all three branches of Government to protect the privacy and civil liberties of US persons."

Critics, however, say that the measure has allowed the NSA to reap communications of Americans, including communications wholly inside the country without a foreign component.

It has also been revealed that the law allowed the collection and retention of incidental personal information unrelated to the targets of investigations.

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The government has also been accused of using information collected in prosecutions of Americans, such as in drug cases.

Senator Ron Wyden, the leading critic of 702 in Congress who serves on the Senate Intelligence Committee, has suggested that the amounts of incidentally collected data on Americans is sizable.

Wyden argues that 702 must be revised to tighten the authorizations and require more protection of Americans from the NSA's spying.

"How many innocent law-abiding Americans have been swept up in this program that has been written and developed to target foreigners overseas?" he wrote in a statement early this year.

"Congress's judgment about the impact of section 702 depends on getting this number."