A group calling itself the "Pakistani Cyber Army" has hacked into the website of India's federal crime investigation bureau and left a warning message to Indian hackers, local media said on Saturday.
The website of the Central Bureau of Investigation (CBI) was penetrated as nuclear-armed rivals India and Pakistan have made efforts to repair relations and build trust that evaporated after the 2008 Mumbai attacks.
The website has remained shut for the day after the action against it on Friday night. The group has warned it would carry out the "mass defacement" of Indian websites, according to the Press Trust of India.
"CBI registered a case for the defacement of its website," the agency said on Saturday. "It has come to the notice of CBI that its official website was unauthorisedly accessed and defaced. A case has been registered in this connection."
South Asian neighbours India and Pakistan have gone to war three times since independence from Britain in 1947 and each remains twitchy about perceived threats from the other.
The Indian government has flagged concerns over its cyber security after a series of attacks on government websites by suspected Chinese hackers, including the office of the National Security Adviser.
The government has threatened to shut off some services offered by BlackBerry-maker Research in Motion , a campaign driven by fears that unmonitored communication puts the country's security at risk. RIM says it is confident there will be no ban on its services.