Pfizer Inc (PFE.N) is the mystery bidder for Swedish Orphan Biovitrum AB (SOBIV.ST), the rare disease specialist which disclosed this week that a potential buyer had made a preliminary offer, people with knowledge of the situation said.

The Swedish company, known as Sobi, has a market value of 35.8 billion Swedish crowns ($4.3 billion). Its medicines for rare or "orphan" conditions make it a target in a consolidating sector in which large, cash-rich drugmakers seek to bolster their portfolios with offerings from smaller biotech firms.

Sobi did not identify its potential acquirer when it revealed on Monday that it had received an approach, but Biogen (BIIB.O) and Pfizer (PFE.N) were viewed by analysts as the most likely bidders, as they already have partnership deals with Sobi.

The sources asked not to be identified because the negotiations are confidential. The price that Pfizer has offered could not be learned. Spokesmen for both Sobi and Pfizer declined to comment.

Pfizer Chief Executive Ian Read said on Tuesday during an earnings call that he was open to deals and was "agnostic" about the size of acquisitions.

Read added that current valuations in the biotech sector were "buoyant", meaning Pfizer's business development team would be very careful as it evaluated opportunities.

Sobi manufacturers a hemophilia treatment called ReFactor AF, which Pfizer sells. On Wednesday, Sobi postponed its annual shareholder meeting, which was scheduled for May 6, in the wake of the takeover proposal.

Pfizer has actively been looking at takeover targets after an attempt to buy AstraZeneca Plc (AZN.L) failed last year. The company agreed to acquire Hospira Inc for $15 billion in February.

Pfizer has established a Rare Disease Research Unit since it sees drugs for rare diseases, which can command very high prices, as an important opportunity in its mix of businesses.

Recent deals involving rare disease companies include Teva Pharmaceutical Industries Inc's (TEVA.TA) acquisition of Auspex Pharmaceuticals for $3.5 billion in March, Shire Plc's (SHP.L) acquisition of NPS Pharmaceuticals for $5.2 billion in January and BioMarin Pharmaceutical Inc's (BMRN.O) acquisition of Prosensa Holding NV for $840 million last November.