The New York Stock Exchange confirmed it had been informed by the government that its website was also subjected to the attack. Other targeted websites included those of the National Security Agency, Homeland Security Department, the Nasdaq stock exchange and the Washington Post, Fox reported.

The Homeland Security Department said there were 5,499 known breaches of US government computers in 2008, up from 3,928 in 2007.

Admiral Michael Mullen, the chairman the Joint Chiefs of Staff, would not discuss the details of the case, but told reporters Wednesday that cyber attacks were an increasing threat and the Pentagon was devoting more resources to counter them.

'We need to have this as a big part of our focus with respect to the threat now and in the future,' he said.

The South Korean government said the websites of the Presidential Office, the National Assembly and Defence Ministry were simultaneously bombarded with access requests.