(Adds statement from NYPD spokesman)

By Joseph Ax

NEW YORK, Feb 11 (Reuters) - New York City's police have made extensive use of covert devices to track cell phones without obtaining warrants since 2008, a civil liberties group said on Thursday, revealing how frequently law enforcement in the largest U.S. city has employed the technology.

The New York Civil Liberties Union released files that showed the New York Police Department used "cell site simulators" to track nearby cell phones more than a 1,000 times over the past eight years.

The American Civil Liberties Union has identified 60 local, state and federal agencies that have adopted the devices in recent years, but the group has said there are likely far more. The extent of the devices has largely been shrouded in secrecy, as departments and private manufacturers such as Harris Corp have refused to disclose information about their use.

The documents released on Thursday were obtained by the NYCLU through a Freedom of Information Law request.

The NYPD does not have a written policy on using the surveillance devices and does not obtain warrants when doing so, according to the NYCLU.

Instead, the department seeks "pen register" orders, which have been used for decades to gather information on specific phone numbers. The orders are issued by judges but require a lower standard than the probable cause needed for warrants.

The NYPD's practice is less stringent than the one adopted last year by the U.S. Department of Justice, which calls for warrants except in emergency situations.

"We still have concerns that this military equipment is being used in a civilian context," said Mariko Hirose, an NYCLU attorney. "At the very least, they should be using warrants and with a strict privacy policy that is written."

The devices mimic cell towers and intercept signals from nearby phones to gather information. That data can include locations of calls, numbers that are called or texted and even the content of communications, the NYCLU said.

The simulators can also sweep up information from nearby "bystander" phones.

J. Peter Donald, a NYPD spokesman, said the department "ensures we have established probable cause, consults with a district attorney, and applies for a court order" before using the devices.

He added that the NYPD does not capture the content of communications or any data from bystander cell phones.

"Perhaps the NYCLU should fact check their press release before issuing it," he said.

U.S. Representative Jason Chaffetz of Utah, a Republican, has introduced a bill to require warrants for the use of cell site simulators.