The possible revision of the offer comes barely two weeks ahead of a shareholders vote on the deal.
Glencore's announcement followed a demand by a Qatari investment fund for better terms for its 10 per cent stake in Xstrata. It wants 3.25 new Glencore shares for each Xstrata share, significantly more than the 2.8 shares currently on offer.
The Financial Times reported Wednesday that the Qatari demand means that holders of about a quarter of Xstrata shares are opposed to the terms, enough to block the deal.
Some shareholders have raised objections to retention provisions in the deal, including a three-year, $45 million deal for Xstrata's chief executive Mick Davis, who is in line to become chief executive of the merged company.
In all, the deal calls for 73 senior employees of Xstrata to share in 173 million pounds of incentives to remain in their jobs.
Glencore said it had received a revised proposal from Xstrata "and will make a further announcement when appropriate."
When the deal was announced in February, two big fund managers — Standard Life Investments and Schroder Investment Management — said Glencore's offer undervalued Xstrata.
The Association of British Insurers also opposes the retention provisions.
Glencore, which has scheduled a general meeting of shareholders for July 11, already holds a 34 per cent stake in the mining company. Xstrata shareholders are to vote on July 12.
Glencore shares were down 2.8 per cent at 294 pence in morning trading in London, while Xstrata's share price fell 1.2 per cent to 776 pence.