Internet retail titan Amazon on Thursday introduced a home virtual assistant always at the ready to answer questions, fetch news, play music or help with to-do lists.

Cloud-based brains referred to as "Alexa" were built into Echo speakers and were seen as a challenge to "Siri" virtual assistants in Apple's coveted mobile devices.

Echo became available on an invitation-only basis at a price of $199, or $99 for those who pay for the Seattle-based company's Prime subscription service.

"Echo's brain is in the cloud, running on Amazon Web Services so it continually learns and adds more functionality over time," Amazon said at a web page where people could request invitations to buy the Bluetooth enabled speakers.

"The more you use Echo, the more it adapts to your speech patterns, vocabulary, and personal preferences."

A demonstration video online showed revealed a female voice responding to questions or commands preceded by the trigger word "Alexa."

A ring of microphones built into can-shaped Echo speakers enable the devices to identify voices even when music is playing, according to Amazon.

Echo accesses news, weather, streaming music and more using wireless Internet connections.

Echo can also be managed remotely using free applications built for smartphones powered by Apple, Android or Amazon's Fire operating software.

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