A major shareholder in Xstrata is demanding better terms in the company's proposed takeover by commodities giant Glencore International.

Qatar Holding, a Qatari government investment fund, said it saw merit in combining the companies, but wanted 3.25 new Glencore shares for each Xstrata share as part of the deal. Glencore originally offered 2.8 shares.

The unexpected demand announced on Tuesday has complicated plans by Glencore and Anglo-Swiss miner Xstrata to close the deal to create a company with a combined value of about $US90 billion ($A89.74 billion).