The voice-activated Siri assistant on Apple''s iPhone 4S is a direct challenge to Google's search engine, chairman Eric Schmidt said. 

"Apple has launched an entirely new approach to search technology with Siri, its voice-activated search and task-completion service built into the iPhone 4S," AFP quoted Schmidt acknowledged as saying in a written response to lawmakers released by the Senate antitrust subcommittee. 

He also pointed to challenges from social networks like Facebook, micro-blogging website Twitter and traditional competitors such as Microsoft's Bing. 

"Social networking sites like Facebook and Twitter also allow users to leverage their social networks to find answers to their questions. Google is therefore competing with all methods available to access information on the internet, not just other general search engines," Schmidt wrote. 

"The source of Facebook's competition with Google is not only through using Bing to search the internet but, also by offering users a fundamentally different way to discover and connect with information on the internet." 

Respected technology site TechCrunch has already described Siri, introduced last month, as Apple's "entry point" into the search engine business, while a Forbes commentator has called the service a "Google killer." 

Schmidt's response comes as Google, which according to internet marketing research firm ComScore has a 65.3 per cent share of the US search engine market, is under investigation by US and European antitrust bodies. 

But he insisted that Google was not "dominant" in internet search, as lawmakers have contended. 

"Google has many strong competitors," Schmidt wrote. "So inferring that Google is in any way 'dominant' in search would be incorrect." 

He also said the company "has none of the characteristics that I associate with market power." 

In September, Schmidt had already told lawmakers that Google was worried customers would quickly switch to other services.