Quoted: A News Sitemap uses the Sitemap Protocol, with the following differences:
...
A second namespace for the News schema should be added:
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Each URL may include optional news-specific tags within the News schema, as described in the table below.
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<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:news="http://www.google.com/schemas/sitemap-news/0.9">
<url>
<loc>http://mynewssite.com/article100.html</loc>
<news:news>
<news:publication_date>2006-08-17T03:19:00Z</news:publication_date>
<news:keywords>Sports, Baseball</news:keywords>
</news:news>
</url>
</urlset>