Google is opening the Google Assistant to outside developers to help the company chase Amazon.com.

On Thursday, the Alphabet unit launched a system for developers to build chatbots that work with Google Assistant, its voice-based virtual helper. The tools, called Conversation Actions, will let companies and other third parties interact with Google users by building bots that answer questions and, eventually, sell and book things through voice controls.

Developers can seek approval for phrases that come after "Ok, Google" to launch the interactive bots. So, in the future if someone says, "Ok, Google, talk to Target," a chatbot for the retailer might appear and help the person buy things through a conversation, for example. They will even be able to choose among four different voices, two male and two female.

Many large technology companies, including Amazon, Apple, Microsoft and Facebook are investing in digital assistants that use artificial intelligence techniques to interact with humans more naturally. With Google Assistant, Google is trying to keep its profitable web search business relevant in an age of mobile devices and wearable gadgets.

At first, the developer function will only work with Google Home, the company's new voice-activated speaker. It will come next to Google's Pixel smartphones and its messaging app Allo, and will soon add support for features like purchases and bookings, Google said.

Amazon has rolled out a similar feature with Alexa, its own virtual assistant, called skills, which links to external apps and services. Alexa works primarily with Amazon's Echo speakers, which compete with Google Home. Thus far, Amazon has created more than 5,000 skills with partners ranging from Domino's Pizza to ride-hailing service Uber.

Google claims that its Conversation Actions are different from Amazon's approach. Rather than picking and setting up skills manually, as Echo owners do, Google imagines its users conjuring any use case with a verbal command -- as long as a developer has built it and claimed the relevant phrase.