Another idea to "force" US hand is to make it their problem in other ways. Dig it up, and threaten to leave it / break it on a place where currents flow towards continental United States.

Of course this should never be done, as it is some sort of eco-terrorism, but is one of the many ways the US can have their hands forced.

Edit: in case this is not clear, I am never advocating for something like this to happen. But it is always a stupid idea to leave potentially lethal and dangerous materials on the land of a place you've scarred and destroyed and which doesn't want it at all.