In a bid to thwart hacking, the government has started keeping secret files in stand-alone computers devoid of any external links and storage devices.

Official sources said the step has been taken after frequent attempts by international hackers, specially from China, to penetrate into computers of sensitive ministries like Defence, External Affairs, Home and Prime Minister's Office.

"Secret files are now kept in computers which have no external links. Besides, no pen drive is allowed to install in such stand-alone computers," a source said.

Investigations by government agencies have found that hackers from China regularly try to penetrate into computers used by top officials and the take away files to four particular destinations in China.

"The Chinese hackers take away around four million data every day from computers across the globe," the source said.

Former National Security Advisor M K Narayanan had said that his office and other government departments were targeted on December 15, 2009 the same date when Google reported cyber attacks originating from China.

He said the attack came in the form of an e-mail with a PDF attachment containing the 'Trojan' virus which allows a hacker to access a computer remotely and download or delete files.

Google had shut down its operations in China after uncovering hacking attempts into e-mail accounts of Chinese human rights activists.