Spy kits that can track mobile phones and intercept calls and messages have been discovered in Washington and beyond, the U.S. government has said.

The Department of Homeland Security (DHS) says it has observed "anomalous activity" consistent with the use of so-called stingrays, BBC reported.

They could be used by foreign spies or criminals, although the DHS said it did not know who was using them.

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It added that such devices pose a "growing risk".

Stingrays, a brand name for a type of International Mobile Subscriber Identity catcher (IMSI), are mobile phone surveillance devices that mimic mobile phone towers.

The size of a briefcase, the devices send out signals to trick mobile phones into transmitting their location and identifying information.

Read alsoRussian news service RT to go off the air in the Washington area – mediaAs well as tracking the mobile phone of a suspect, the devices also gather information about phones of bystanders who are nearby.

It is believed to be the first time the U.S. government has acknowledged the use of rogue spying devices in Washington.

The revelation came in response to a letter from U.S. Senator Ron Wyden to the DHS, asking about the unauthorized use of such devices.

The agency response was obtained by the Associated Press from Wyden's office.

Read alsoU.S. plans to sanction Russian oligarchs this week: mediaThe use of Stingray devices by police forces across the U.S. is being tracked by the American Civil Liberties Union (ACLU). It has identified 73 agencies in 25 states that own such devices but believes there could be many more in use which are not formally declared.

There are concerns among politicians in Washington that such devices could also be used by unauthorized agencies, such as foreign governments.