Google Inc.'s Nest Labs said it agreed to buy video-monitoring and security startup Dropcam Inc. for $555 million as part of a push to become the dominant operating system for connected devices in and around the home.

Nest, which makes Internet-connected thermostats and smoke detectors, is paying cash for the startup and will work with Dropcam to develop products and services that connect users to their homes, Matt Rogers, co-founder of Nest, said in an interview. The deal requires regulatory approval.

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