* Extension of law is "top priority" for intelligence agencies
* Senate critics threaten delays over civil liberties concerns
* Secret program sorts through masses of phone calls and emails
WASHINGTON, Sept 11 (Reuters) - U.S. intelligence officials made a public plea on Tuesday, the 11th anniversary of the Sept. 11, 2001, attacks, for quick congressional action to extend a sweeping but controversial U.S. electronic surveillance law.
Robert Litt, chief lawyer for the Office of Director of National Intelligence, told reporters that winning congressional approval to extend the electronic spying law was the U.S. intelligence community's "top priority."
If the law, which expires at the end of 2012, is not extended, Litt said, U.S. spy agencies would lose access to what he described as a "very, very important source of valuable intelligence information."
Relevant committees of both the House of Representatives and the Senate have approved similar, though not identical, versions of bills that would extend the surveillance law, an updated version of the Foreign Intelligence Surveillance Act, or FISA.
The Senate Intelligence Committee's version would extend it until 2017. A Senate Judiciary Committee version would extend it only until 2015.
Some congressional officials said the Obama administration was anxious to get an extension of the law approved by Congress in the next two weeks, since legislators adjourn for an election break later this month and considerable unfinished business already awaits them for a lame duck session after the Nov. 6 general election.
But at least one congressional critic of the surveillance law says he is willing to use legislative tactics to stall the bill unless the administration and other legislators agree to include stronger provisions to protect Americans' civil liberties.
Senator Ron Wyden, a Democratic member of the Senate Intelligence Committee, said he had placed a "hold" on the bill that he would not lift until the Senate considers more stringent protections against warrantless spying on Americans. "My hold is on and it will stay on," he told Reuters.
Wyden said that in correspondence with a group of senators, the Obama administration had admitted that some Americans' rights prohibiting warrantless surveillance had been violated by the spying program. He said that until loopholes in the law were plugged, he believed it should only be extended for a relatively short period.
BILL CONTAINS LOOPHOLE
In his conference call with reporters, Litt declined to discuss details of how U.S. agencies, most notably the ultra-secret National Security Agency, the electronic eavesdropping organization based at Fort Meade, Maryland, go about collecting information under the act's provisions.
The law authorizes broad electronic intelligence collection by U.S. agencies targeting what a Senate report described as "persons reasonably believed to be located outside the United States." Under its provisions, several officials said, U.S. agencies do not have to obtain a court warrant to monitor communications of suspected militants or other intelligence targets who are not located in the United States.
Wyden and Democratic Senator Mark Udall alleged earlier this year that the bill contained a "loophole" that could be used "to circumvent traditional warrant protections and search for the communications of a potentially large number of Americans."
Wyden and Udall proposed an amendment that would have ordered Inspectors General at the Justice Department and National Intelligence Director's office to produce a "rough estimate" of how many Americans' communications had been inadvertently collected under the law.
On Tuesday, Litt maintained that because of the way the collection program worked, producing such an estimate would be impractical.
People familiar with the program said that it involved sifting through masses of communications between foreigners that are transmitted via servers or telecommunications links that pass physically through the United States.
One official familiar with the matter said that the only way to begin to estimate the extent to which the program might have inadvertently collected information on Americans is by looking more closely at messages that intelligence officials are not supposed to look at - because Americans are on one or both ends of the messages.
One of the main points of the law authorizing the surveillance program is that officials are not supposed to be reading or listening to message traffic involving people located in the United States unless they have warrants to do so.
Steven Aftergood, a secrecy expert with the Federation of American Scientists, said there ought to be a "middle ground" under which spy agencies could provide Congress with an estimate of the "magnitude" of inadvertent collection on Americans without compromising details of the system.
"Are we talking about ten, or ten million, or ten billion" inadvertently collected messages involving U.S. people, Aftergood said. But such an estimate, he said, is "what the government is refusing to provide."