US cyber intelligence experts suspected that a spate of attacks on the computer networks of White House and Pentagon was carried out by Chinese hackers but conceded that it was difficult to trace the exact source, a media report said in New York on Saturday.

The Financial Times quoted senior officials as saying that Chinese hackers had penetrated the White House computer network on multiple occasions and obtained e-mails between government officials.

"We are getting very targeted Chinese attacks so it stretches credulity that these are not directed by government-related organisations," US government's cyber intelligence experts were quoted as saying.

They, however, conceded that it was extremely difficult to trace the exact source of an attack beyond a server in a particular country, the Times said.

On each occasion, the paper said that the cyber attackers accessed the White House computer system for brief periods, allowing them enough time to steal information before computer experts from the US patched the system.

The report also said that the US government cyber investigators have determined that an attack on Obama and McCain campaign computer networks also originated in China.

Details of the intrusion were first reported by Newsweek.

According to the Newsweek report, the Obama campaign speculated that China or Russia were behind the attacks. A second official was quoted as saying that cyber analysts had concluded that the attacks originated in China, but stressed that they were not able to determine who was responsible.

White House press secretary Dana Perino declined to comment. The Chinese embassy also did not comment, but in the past China has called similar allegations reflective of 'Cold-War thinking'.

The report quoted officials as saying that the Chinese cyber attacks had the hallmarks of the 'grain of sands' approach taken by its intelligence, which involves obtaining and pouring through lots of, often low-level, information to find few nuggets.

Some US defence companies have privately warned about attacks on their systems, which they believe are attempts to learn about future weapons systems, the Times said.

The National Cyber Investigative Joint Task Force, a new unit established in 2007 to tackle cyber security, detected the attacks on the White House.

But the report said official stressed that the hackers had only accessed the unclassified computer network, not the more secure classified network.

"For a short period of time, they successfully breach a wall, and then you rebuild the wall, it's not as if they have continued access," said the official.

The US has increased efforts to tackle cyber security, particularly since Chinese hackers believed to be associated with the Peoples' Liberation Army last year perpetrated a major attack on the Pentagon.

The cyber attackers managed to obtain information and emails traffic from the unclassified computer system that supports Robert Gates, the defence secretary.

Pentagon IT technicians were forced to take the network down for days to conduct repairs, the Times said.

Concerns about Chinese hacking last year prompted President George W Bush to tell reporters ahead of a meeting with President Hu Jintao of China that he might raise the issue with countries of concern.

Over the past year, the US government has tightened restrictions on officials using BlackBerrys and computers overseas, particularly in Russia and China, and sometimes bars them from removing the equipment from US government aircraft in the country, the report added.