Chinese hackers have penetrated the White House computer network on multiple occasions, and obtained e-mails between government officials, a senior US official told the Financial Times.

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

US government cyber intelligence experts suspect the attacks were sponsored by the Chinese government because of their targeted nature. But they concede that it is extremely difficult to trace the exact source of an attack beyond a server in a particular country.

”We are getting very targeted Chinese attacks so it stretches credulity that these are not directed by government-related organisations,” said the official.

The official said the Chinese cyber attacks had the hallmarks of the “grain of sands” approach taken by Chinese intelligence, which involves obtaining and pouring through lots of - often low-level - information to find a 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 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 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 is not as if they have continued access,” said the official. ”It is constant cat and mouse.”

Dana Perino, White House press secretary, 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 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.

US military computer experts battled for weeks against a sustained attack that eventually overcame the Pentagon’s defences. 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.

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.

In another incident, US government cyber investigators have determined that an attack this summer on the Obama and McCain campaign computer networks also originated in China. Details of the intrusion were first reported by Newsweek.

The Secret Service warned the Obama and McCain campaigns their networks had been comprised. The hackers successfully downloaded large quantities of information, which security agencies believed was an attempt to learn more about the contenders’ policy positions.

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

”There is no doubt that foreign governments are actively targeting cyber space not only for sensitive information but to influence our most sensitive processes such as the US presidential election,” said Sami Saydjari, head of the Cyber Defence Agency, a private company that advises government on hacking.

“This underscores the need for President-elect Obama to take leadership in the cyber space race that is well underway.”

While the US has raised concerns about cyber attacks, many governments believe the US is also engaged in electronic spying. Bob Woodward, the veteran Washington Post reporter, this year revealed that the US had been spying on the Iraqi government.