APThe attackers focused on governments, corporations and journalists interested in China.
The group remained undetected for so long as it was able to re-use methods and malware dating back to 2005.
Hackers, most likely from China, have been spying on governments and businesses in Southeast Asia and India uninterrupted for a decade, researchers at internet security company FireEye Inc said.
In a report released on Monday, FireEye said the cyber espionage operations dated back to at least 2005 and "focused on targets - government and commercial - who hold key political, economic and military information about the region."
Bryce Boland, Chief Technology Officer for Asia Pacific at FireEye and co-author of the report, said the attack was still ongoing, noting that the servers the attackers used were still operational, and that FireEye continued to see attacks against its customers, who number among the targets.
China has always denied accusations that it uses the Internet to spy on governments, organisations and companies. Neither the Foreign Ministry nor the Cyberspace Administration of China, the Internet regulator, immediately responded to written requests for comment on the FireEye report on Monday.