Putin is talking about a new missile system, not necessarily a new warhead
Gary discusses this story in a thread higher on the page. The key development is MaRV technology, perhaps combined with a penetrator warhead. However, that penetrator capability can be developed without having to detonate nukes. The US is also considering developing a new bunker buster case for an existing warhead design. For certain scenarios (e.g. North Korea), such a capability might be needed should things take a drastic turn for the worst. We're also working on a 30,000 pound conventional explosive bunker buster bomb.