Cyber-attacks traced to China targeted at least 48 chemical and military-related companies over the summer in an effort to steal technical secrets, an American computer security company has said, raising the temperature of complaints about pervasive internet crime emanating from the new superpower..

The targets included 29 chemical companies and 19 others which make advanced materials used by the military, said security firm Symantec in a report. It said the group included multiple Fortune 100 companies but did not identify them or say where they were located.

"The purpose of the attacks appears to be industrial espionage, collecting intellectual property for competitive advantage," said the report.

The claim comes as a conference meets in London to discuss cybercrime and internet attacks, led by the defence secretary William Hague, and after the chief of the GCHQ listening agency warned that threats against government infrastructure from foreign hackers were at "disturbing" levels.

Security experts say China is a centre for internet crime, whether government-sanctioned or not. Multiple attacks against governments, companies and human rights groups have been traced to this country, though finding the precise source is nearly impossible. China's military is a leader in cyberwarfare research but the government has rejected allegations of cyber-spying, and says it also is a target.

The latest attacks occurred between late July and September and used emails sent to companies to plant software dubbed "PoisonIvy" in their computers, Symantec said. It said the same hackers also were involved in attacks earlier this year on human rights groups and auto companies.

Symantec said it traced the attacks to a computer system owned by a Chinese man in his 20s in the central province of Hebei. It said that when contacted, the man provided a contact who would perform "hacking for hire."

The company said it could not determine whether the Chinese man was a lone attacker, whether he had a direct or indirect role or whether he hacked the targets for someone else. It called him "Covert Grove" based on a translation of his Chinese name.

Security consultants say the high skill level of earlier attacks traced to China suggests its military or other government agencies might be stealing technology and trade secrets to help state companies.

The chairman of the US House of Representatives Intelligence Committee, Mike Rogers, said last month that Chinese efforts to steal American technology over the internet had reached an "intolerable level." He called on the US and other governments to pressure Beijing to stop.

The claims by Symantec add to the drumbeat of intensifying use of the internet for industrial and military espionage between nations. China was suspected of being behind the "Titan Rain" attacks which in 2006 targeted the US military and UK parliament, and earlier this year of an attack on the International Monetary Fund, which apparently was hacked to uncover secrets about money flows.

Equally, western intelligence agencies are suspected of having created the "Stuxnet" computer worm to target nuclear centrifuges in Iran and delay that country's efforts to acquire a nuclear capability.

In August another security firm, McAfee, said it had found a five-year-long hacking campaign that it called Operation Shady Rat against more than 70 governments, international institutions, corporations and thinktanks.

In February, McAfee said hackers operating from China stole information from oil companies in the US, Taiwan, Greece and Kazakhstan about operations, financing and bidding for oil fields.

Thousands of Chinese computer enthusiasts belong to hacker clubs, and experts say some are supported by the military to develop a pool of possible recruits. Experts say military-trained civilians also might work as contractors for companies that want to steal technology or business secrets from rivals.

China has the world's biggest population of internet users, with more than 450 million people online, and the government promotes web use for business and education. But experts say security for many computers in China is so poor that they are vulnerable to being taken over and used to hide the source of attacks which might originate elsewhere.

Last year, Google closed its China-based search engine after complaining of cyber-attacks from China against its Gmail email service in which Chinese dissidents were targeted.

That case highlighted the difficulty of tracking hackers. Experts said that even if the Google attacks were traced to a computer in China, it would have to be examined in person to be sure it wasn't hijacked by an attacker abroad.