BHP was struggling to find common ground with Anglo American on Tuesday in talks over its takeover offer, with no new concessions as a deadline nears for the world's biggest miner to submit a binding offer, five sources said.

Anglo granted its bigger rival a one-week extension until 1600 GMT on Wednesday to its original May 22 deadline to submit a binding offer, after rejecting a third takeover proposal that had been dismissed as difficult to execute. It agreed to hold talks with BHP to iron out contentious issues over the structure of the deal.

The latest BHP offer values Anglo at £29.34 a share or £38.6-billion ($49.38-billion) and is contingent on Anglo unbundling its South African platinum and iron ore assets - Anglo American Platinum and Kumba Iron Ore.

Reuters spoke to five of Anglo's top 20 investors who had calls with BHP after its third offer was rejected. The investors said that BHP at present is maintaining that it wouldn't be amending the value and structure of the deal.

Anglo is sticking to its position that the offer isn't compelling enough and that BHP's proposed structure is difficult to implement and erodes value, the sources said.

"Anglo is talking to BHP but I am not sure if it's just going through the motions so it can say it tried, or genuinely tried to get somewhere," Ian Woodley, a portfolio manager at Old Mutual said. "BHP said it won't change its structure and that it can't or won't take over Anglo as it is because the unbundling of Amplats and Kumba just gets too complicated then."

BHP and Anglo declined to comment.

Anglo has itself outlined a plan to divest its less profitable coal, nickel, diamond and platinum units to focus on expanding copper output to more than 1 million tons in a decade.

Anglo shares in London closed down 2.1% at 25.58 pounds, a discount of about 15% to BHP's last offer.

Some investors have said they would have preferred an option whereby BHP offered to buy the whole company and spin off the assets it doesn't want afterwards.

"We asked BHP, if you think it's that easy (to demerge the South African assets), why don't you buy the company in full," one investor said. "They can't really answer that, they just say it is not aligned with their strategy."

Another investor said they asked BHP CEO Mike Henry if he isn't concerned about risking the deal over the refusal to yield on the South African assets, which would represent less than 10% of its entire portfolio, if the transaction is successful.

Henry responded that he was also considering feedback from his investors who have warned him against changing the structure, or raising the offer again.

A source familiar with the matter said that Anglo isn't going to extract any more concessions from BHP.

BHP sees its offer as "very generous, especially if you imagine there are a lot of synergies," the source added.

Another source said that Anglo could extend the deadline for BHP to submit its offer if there was a shift in positions on either side.