Comcast Corp.’s CMCSA, +1.09% NBCUniversal and Alphabet Inc.’s GOOGL, -0.05%  Google have emerged as top contenders to work with Netflix Inc. NFLX, +4.67%  and help the streaming company create an advertising-supported tier of its service, according to people familiar with the matter.

Netflix, which is hoping to boost revenue by selling ads around its programming, is still in the early stages of developing the strategy and has explored a range of tie-ups in recent weeks.

A partnership with NBCUniversal would likely be exclusive, the people familiar with the matter said. Comcast’s video ad unit, FreeWheel, would supply technology to help serve up ads, while NBCUniversal’s ad-sales team would help sell ads in the U.S. and Europe, the people said.

A partnership with NBCUniversal would likely involve revenue-sharing, and one issue might be whether Netflix would be guaranteed a certain amount of revenue, they said. Linda Yaccarino, chairman of global advertising and partnerships for NBCUniversal, would be a major player in such a partnership.

An expanded version of this story appears on WSJ.com.

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