Google is investigating whether Chinese employees helped in a cyber-attack on the internet search giant last month.

The firm said last week it was thinking of pulling out of China after a “sophisticated” attack on its network that resulted in theft of its intellectual property.

Sources said the attack, which targeted people who have access to certain parts of Google networks, may have come from Google China's office.

“We're not commenting on rumour and speculation. This is an ongoing investigation, and we simply cannot comment on the details,” a Google spokeswoman said.

Google claims the attack targeted at least 20 other companies. Adobe Systems Inc, Juniper Network Inc and Rackspace Hosting Inc have acknowledged being targets and a person familiar with the situation said Yahoo Inc was also a picked out.

Security analysts said the programme used in the Google attack was a modification of a trojan — a programme that allows a hacker unauthorised access to a computer — called Hydraq. Analysts said the sophistication in the attack was in knowing who to attack.

Sources also said some Google China employees were denied access to internal networks after 13 January while some staff were put on leave and others transferred to different offices in Google's Asia Pacific operations. Google declined to comment.