Comcast Corp.'s NBCUniversal has upped its financial stake into BuzzFeed with an additional $200 million, bringing its total investment into the digital news site to $400 million.

NBCUniversal also disclosed a partnership in which the two companies, one old media and one new media, will cooperate on selling advertising. NBCUniversal can offer advertisers inventory on BuzzFeed, while BuzzFeed can offer inventory on NBCUniversal.

NBCUniversal  disclosed the deal on Monday, noting that it was seeking to broaden its millennial audience. The two companies also will collaborate on short-form video for advertisers.

NBCUniversal, one of the nation's biggest entertainment and news conglomerates, had previously put $200 million into BuzzFeed. The privately held news site is eventually expected to sell shares to the public. NBCUniversal's investment is not a controlling or majority stake, BuzzFeed says.

BuzzFeed says it reaches 500 million people around on digital platforms.