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His comments come shortly after Washington signed a deal with Kyiv to share the profits of Ukraine's rare earth minerals in return for future US security assistance.

Details are still emerging about the agreement, but both sides have confirmed that it sets up an investment fund to search for minerals, and set outs how revenues would be split.

In December, China banned the export of some rare earth minerals to the US, having previously limited mineral exports to the US the previous year.

He has been on the hunt for ways to mine rare earth minerals, which China effectively has a monopoly over, making this a key part of any deal with Ukraine.

Both the US and Ukraine have said they are broadly ready to sign a deal which would give American firms access to Ukraine's minerals.