LONDON - The West's push to build a home-grown magnets supply chain to reduce its reliance on China - led by massive U.S. backing for Nevada-based MP Materials - is running into a critical problem: the scarcity of so-called heavy rare earth elements.

The United States and allies have been scrambling to create an alternative supply chain to make super-strong rare earth magnets, which are vital components in everything from defence technology and electric vehicles, to electronics and wind turbines.