Google has admitted that testing of its controversial social network Buzz was insufficient.

The firm has had to make a series of changes to the service after a ferocious backlash from users concerned about intrusions of privacy.

Buzz was only tested internally and bypassed more extensive trials —used for many other Google services — with external testers.

Google said that it was now working "extremely hard" to fix the problems.

"We're very early in this space. This was one of our first big attempts," Todd Jackson, Buzz product manager, told BBC News.

"We've been testing Buzz internally at Google for a while. Of course, getting feedback from 20,000 Googlers isn't quite the same as letting Gmail users play with Buzz in the wild."

Many of the firm's new services are tested by the so-called Google Trusted Tester program, a network of friends and family of Google employees who are given confidential access to products before they launch.

But Buzz was not tested by this program.

The firm has now set up a "war room" at Google HQ to bring together engineers and product managers to make decisions about what changes need to be made to Buzz.

"If it becomes clear that people don't think we've done enough, we'll make more changes," said Jackson. "We know we need to improve things," he continued.

One problem that immediately caused concern was Google's decision to automatically give users a ready-made circle of friends based on the people they most frequently e-mailed.

Unless users changed settings in their profile, this list could automatically be made public, allowing anyone to see who a user corresponded with most frequently.

Other possible changes include a better "preferences menu" that will allow users to better tailor what appears in their inbox, and a more prominent "mute" option to switch the service off.

The most recently announced changes would go live "this week," he added.