WASHINGTON — Sen. Ron Wyden, D-Ore., will seek to block passage of an intelligence bill that extends the government’s eavesdropping authorities because the intelligence community won’t say how many Americans are being monitored, he said Tuesday.

At issue is the Foreign Intelligence Surveillance Act, which was passed in 1978 in response to revelations of political wiretapping.

The law was updated in 2008 under President George W. Bush in a way that essentially legalized the “warrantless wiretapping” program aimed at stopping terror plots. The intelligence bill, approved Monday by the Senate’s Select Committee on Intelligence, would extend the 2008 changes until 2015.

Those changes greatly expanded the government’s surveillance authorities. The targets must be a foreigner out of the country, but their conversations with Americans are fair game.

After first opposing them, then-Sen. Barack Obama voted for the 2008 FISA changes, which gave legal immunity to telecom companies that cooperated with Bush’s spying program. He said he became convinced the capabilities were needed to hunt for terrorists.

Said Wyden, in a statement: “Congress passed the FISA Amendments Act in 2008 in an effort to give the government new authorities to conduct surveillance of foreigners outside the United States.

“The bill contained an expiration date of December 2012, and the purpose of this expiration date was to force members of Congress to come back in a few years and examine whether these new authorities had been interpreted and implemented as intended,” Wyden wrote.

“I believe that Congress has not yet adequately examined this issue, and that there are important questions that need to be answered before the FISA Amendments Act is given a long-term extension.”

Wyden and Sen. Mark Udall, D-Colo., who serve on the intelligence committee, asked the Director of National Intelligence last month how many Americans have had their communications monitored under the law. The DNI’s office responded in a letter that “it is not reasonably possible to identify the number of people located in the United States whose communications may have been reviewed under the authority.”

Wyden also wants to know to what extent the government is tracking the location of Americans using data from their cell phones.

Whether intelligence agencies are doing that domestically is an open question.

At a Senate hearing on July 26, Wyden, who serves on the intelligence committee, asked Matthew Olsen, NSA’s general counsel and the nominee to direct the National Counterterrorism Center, whether government agencies “have the authority to use cell-site data to track the location of Americans inside the United States for intelligence purposes.”

Olsen replied, “There are certain circumstances where that authority may exist.”

He did not elaborate.