We all know that the US law enforcement agencies have been trying to remove the networks of compromised computers used by cyber criminals, but we did not know that US the National Security Agency is now hijacking the botnets and using them for spying.

A slide leaked by former NSA contractor Edward Snowden and published by The Intercept news website on Wednesday has revealed that more than 140,000 computers across the world had been 'co-opted' by the NSA since August 2007 with an aim to inject spying software inside them.

Cyber criminals use botnets to steal financial information from infected machines, to relay spam messages, and to conduct "denial-of-service" attacks against websites.

In November, Federal Bureau of Investigation Director James Comey told the Senate that botnets had "emerged as a global cyber security threat" and that the agency had developed a "comprehensive public-private approach to eliminate the most significant botnet activity and increase the practical consequences for those who use botnets for intellectual property theft or other criminal activities."

The latest slide revealed that NSA used a technique called QUANTUMBOT, which "finds computers belonging to botnets, and hijacks the command and control channel." The NSA had described the program as "highly successful."

In May, it was reported that botnets were tapped by the US agencies to harvest data from the owners of the machines and to develop the ability to issue new commands to the infected computers.

The Top Secret slide revealed by Snowden, a former NSA contractor, was marked for distribution to the "Five Eyes" intelligence alliance, which includes the United States and Britain.

The NSA, however, refused to give any concrete answer over the latest revelation. In a written statement, an NSA spokeswoman said: ?"s the President affirmed on 17 January, signals intelligence shall be collected exclusively where there is a foreign intelligence or counterintelligence purpose to support national and departmental missions, and not for any other purposes.

"Moreover, Presidential Policy Directive 28 affirms that all persons - regardless of nationality - have legitimate privacy interests in the handling of their personal information, and that privacy and civil liberties shall be integral considerations in the planning of US signals intelligence activities," added the statement.