Imagine if China seized a US ship carrying nuclear weapons components. It'd be war immediately, and the US of course knows that the same is true of China.
There's also no basis under international law. China is of course a nuclear weapons state and numerous nuclear weapons states have transported nuclear weapons around the world, with nuclear weapons sharing being a something old and accepted.
One special reason that one would expect China to respond to such an operation by actually going to war immediately is that, if it were a nuclear weapon, with all the details and delivery systems etc., it could be analyzed to see how it reacts to missile defence stuff, which might reveal some unknown weakness that would allow MAD to be ended. So somebody getting themselves access to the core reentry, MIRV and maneuvering stuff would be a reason to attack him right away.