Google has unveiled an upgrade to the way it interprets users' search requests.

The new algorithm, codenamed Hummingbird, is the first major upgrade for three years.

It has already been in use for about a month, and affects about 90% of Google searches.

At a presentation on Thursday, the search giant was short on specifics but said Hummingbird is especially useful for longer and more complex queries.

Google stressed that a new algorithm is important as users expect more natural and conversational interactions with a search engine - for example, using their voice to speak requests into mobile phones, smart watches and other wearable technology.

Hummingbird is focused more on ranking information based on a more intelligent understanding of search requests, unlike its predecessor, Caffeine, which was targeted at better indexing of websites.

It is more capable of understanding concepts and the relationships between them rather than simply words, which leads to more fluid interactions. In that sense, it is an extension of Google's "Knowledge Graph" concept introduced last year aimed at making interactions more human.

In one example, shown at the presentation, a Google executive showed off a voice search through her mobile phone, asking for pictures of the Eiffel Tower. After the pictures appeared, she then asked how tall it was. After Google correctly spoke back the correct answer, she then asked "show me pictures of the construction" - at which point a list of images appeared.

Big payoffs?

However, one search expert cautioned that it was too early to determine Hummingbird's impact. "For me this is more of a coming out party, rather than making me think 'wow', said Danny Sullivan, founder of Search Engine Land.

"If you've been watching this space, you'd have already seen how they've integrated it into the [predictive search app] Google Now and conversational search.

"To know that they've put this technology further into their index may have some big payoffs but we'll just have to see how it plays out," Mr Sullivan said.

The news was announced at an intimate press event at the Silicon Valley garage where founders Sergei Brin and Larry Page worked on the launch of the search engine, which is fifteen years old on Friday.

At the event, the search behemoth also announced an updated search app on Apple's iOS, as well as a more visible presence for voice search on its home page.