A spike in suspicious cell phone activity close to the White House and the Pentagon has raised fresh fears that the US government is being spied on by a foreign power.

The Department for Homeland Security has been alerted to unusual activity around cellphone towers in Washington DC.

A source at security company ESD America has told CBS it could reveal espionage attempts from outside governments.

The activity, the source claimed, could be evidence that specific individuals, or their devices, are being monitored.

The activity, a source claimed, could be evidence that specific individuals, or their devices, are being monitored

A report in the Washington Free Beacon says diplomats and US government officials would be the likely target.

Documents passed to the Free Beacon suggest devices could be cloned, and location data has been tracked by a third party.

A source told the publication: 'The attack was first seen in D.C. but was later seen on other sensors across the USA. A sensor located close to the White House and another over near the Pentagon have been part of those that have seen this tracking.'

Documents suggest devices could be cloned, and location data has been tracked by a third party

Democratic lawmakers have written to Homeland Security Secretary John F Kelly (pictured, right, alongside President Trump and Vice President Mike Pence) raising concerns about hacking of cellphone networks

Insiders believe that a large number of cell phones are being tracked, and rogue forces could introduce malware to spy on sensitive targets.

It comes amid widespread concerns over the potential to hack US cellular networks.

Democratic lawmakers Ron Wyden and Red Lieu wrote this week in a letter to the Homeland Security Secretary John F. Kelly: 'For several years, cyber security experts have repeatedly warned that US cellular communications networks are vulnerable to surveillance by foreign governments, hackers, and criminals exploiting vulnerabilities in Signaling System 7 (a set of protocols used by cellphone and text messaging applications).

'US cellular phones can be tracked, tapped, and hacked—by adversaries thousands of miles away—through SS7-enabled surveillance,' the congressmen write.

'We are deeply concerned that the security of America’s telecommunications infrastructure is not getting the attention it deserves.

'We suspect that most Americans simply have no idea how easy it is for a relatively sophisticated adversary to track their movements, tap their calls, and hack their smartphones.'