Google sitemap namespace confusion

Here the structure of my sitemap:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://ift.tt/xwbjRF">
    <sitemap>
        <loc>somefile.xml</loc>
    </sitemap>
</sitemapindex>

somefile.xml

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://ift.tt/1oM5TQy" xmlns:xhtml="http://ift.tt/lH0Osb">
<url>
  <loc>[some url]</loc>
  <link rel="alternate" hreflang="en" href="[some url]" />
  <link rel="alternate" hreflang="en" href="[some url]?lang=en" />
  <link rel="alternate" hreflang="fr" href="[some url]?lang=fr" />
  <priority>1.00</priority>
</url>

Here is Google's error message:

Your Sitemap or Sitemap index file doesn't properly declare the namespace. Expected: http://ift.tt/xwbjRF Found: http://ift.tt/lH0Osb Tag: urlset

I guess I'm missing something, information on the Internet are confusing. What's is the right syntax? Thank you if you can help.