The agency won the work after a three-way pitch. It is being charged with promoting across the US, particularly the range of international news, analysis and information it offers 24 hours a day.
Gavin Marshall, business director at Agency Republic, said: "BBCnews.com has unparalleled breadth and depth of coverage, but our challenge in the US is to encourage trial among users who are likely to have an established online news behaviour with other providers."
BBC World Service's websites attract 280m page impressions a month. The service also broadcasts in 43 languages on the radio to an audience of 146m.
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