By Paresh Dave Inc (AMZN.O) and Alphabet Inc's (GOOGL.O) Google need to offer more details about how their smart-home devices and virtual assistants will support competition and user privacy, U.S. Senator Amy Klobuchar wrote to the companies on Tuesday.

In a letter, the chair of the Senate Judiciary Committee's antitrust subcommittee said testimony last week by attorneys from the companies left her with concerns about their dominance of the fast-growing field.

She asked the companies which of their products will support - and which will not - a recently revamped industry alliance known as Matter. The group, which includes Apple Inc (AAPL.O), Ikea and others, aims to allow home-automation gadgets such as internet-connected lights and speakers from various companies to sync with one another.

"For what period of time do you commit to support the Matter interoperability project, and who at your companies is responsible for determining whether to extend the length of your commitment to Matter?" Klobuchar wrote to Amazon and Google.

Last week's hearing followed complaints by Sonos Inc (SONO.O) and other home-device makers about big tech companies engaging in allegedly anticompetitive tactics to extend their dominance in advertising and other businesses.

Klobuchar noted at the hearing that Amazon's Echo line had more than 50% of the smart-speaker market, while Google's Nest products had 30%.