LONDON/FRANKFURT (Reuters) - Fresenius (>> Fresenius Medical Care AG & Co. KGaA) is considering pulling out of the bidding for French food group Danone's (>> DANONE) medical nutrition unit because of frustration with the slow progress of the process, two sources familiar with the situation said.

A third source close to the situation said on Friday "the situation is difficult", but added that it was too early to say a decision had been taken to walk away: "It would be wrong to say Fresenius is not interested any more."

The sources said German healthcare group Fresenius was unhappy with the complexity of the sale process for the asset, worth an estimated 4 to 5 billion euros (3.18 to 3.98 billion pounds).

Two of the sources said deadlines had repeatedly been missed by Danone, the maker of Activia yoghurt and Evian water.

The third source added that differences remained in price expectations and said Fresenius was still working with private equity firms Permira [PERM.UL] and PAI, with whom it had submitted a joint bid.

Fresenius, Danone and Permira declined to comment while PAI has repeatedly refused to comment on the matter.

A sale of its medical nutrition business would help Danone fund a takeover of U.S. infant formula maker Mead Johnson Nutrition Group (>> Mead Johnson Nutrition CO), which sources familiar with the matter have told Reuters it wants to pursue.

A fourth source close to the matter said Danone was keen to agree a sale this year but that this would not be possible due to complications about how the deal would be structured since Fresenius would likely have to sell parts of the business for antitrust reasons.

One of the first two sources said Fresenius could inform Danone of a decision to pull out as early as this weekend.

(Reporting by Sophie Sassard, and Ludwig Burger and Arno Schuetze; Additional reporting by Geert de Clerq in Paris; Writing by Georgina Prodhan; Editing by Stephen Brown)