Japanese automakers Toyota and Mazda are forming a partnership to build a new final assembly plant in the United States, with sources familiar with the plan telling CNBC the joint venture and assembly plant will be announced Friday in Japan.

A location for the $1.6 billion facility has not yet been picked, but it's expected to have the capacity to build 300,000 vehicles annually while employing 4,000 workers, the sources said.

The companies hope to open the plant in 2021, the sources said.

The new facility is just one aspect of the partnership Toyota and Mazda, with the two also planning to work together to develop electric vehicles and connected-car technologies, the sources said.

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