Singapore's four telecom companies, Singtel STEL.SI, StarHub STAR.SI, M1 and Simba Telecom TUA.AX, were targeted by cyber espionage group UNC3886 in attacks last year, Singapore's Cyber Security Agency said on Monday.

The agency said the hackers were able to penetrate and gain access to some parts of the telecom systems, but could not disrupt services and did not access personal data.

"They also managed to exfiltrate a small amount of technical data; this is believed to be primarily network-related data to advance the threat actors’ operational objectives," the agency said.

Google-owned cyber security firm Mandiant has described UNC3886 as a "China-nexus espionage group" that has attacked defence, technology and telecommunications organizations in the U.S. and Asia.

Beijing routinely denies any allegations of cyber espionage, and says it opposes all forms of cyberattacks and is in fact a victim of such threats.

The Chinese Embassy in Singapore did not immediately respond to a request for comment.

Monday's statement was the first time the government has revealed the type of infrastructure that UNC3886 targeted. Last July the government said it was responding to cyberattacks from the group targeting high-value strategic assets.

-Reporting by Jun Yuan Yong; Editing by John Mair/Reuters