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LONDON, March 25 (Reuters) - German insurer Allianz is nearing a 2.5 billion euro ($2.94 billion) deal to buy Aviva's Polish unit, trumping rival bids from Italy's Generali and Dutch insurer NN, sources familiar with the matter told Reuters.

Allianz is close to entering exclusive talks with Aviva and a deal could be announced as soon as Friday, the sources said, speaking on condition of anonymity.

A spokesperson for Aviva confirmed the British insurer was exploring options for its Polish business.

Allianz and Generali declined to comment while NN was not immediately available.

The German firm prevailed in a competitive auction that kicked off last year when Aviva's boss Amanda Blanc embarked on an ambitious plan to sell assets across Europe and shift the company's focus to its core operations in Britain, Canada and Ireland.

Aviva is the second-largest life insurer in Poland after state-owned PZU and the tenth biggest provider of general insurance in the country. Its Polish business also includes two key bancassurance partnerships with Santander and ING.

Allianz, which recently missed out on a deal to buy Aviva's French operations, submitted the highest bid for the British insurer's Polish unit - a key asset to grow its influence across central and eastern Europe, the sources said.

Allianz already has a presence in both life and general insurance in Poland as well as businesses in various eastern European markets including Russia and Ukraine.

Allianz, which first entered the Polish market in 1997, ranks as Europe's biggest insurer and while it has no plans to grow its life insurance operations, it sees the deal as a springboard to further expand across lucrative central and eastern European markets, the sources said.

Generali and NN came close to matching Allianz's offer after Aviva gave bidders a chance to sweeten their binding bids which were initially submitted on March 22, the sources said.

"Aviva is the real winner in this auction," one of the sources said, adding that the London-listed insurer had lured bidders to pay up using the strategic geographical positioning of the asset.

Panmure Gordon analysts initially estimated the sale of Aviva's remaining European assets in Poland and Italy would be worth 2.1 billion pounds ($2.9 billion) overall.

Aviva sold its life insurance businesses in Italy to France's CNP Assurances for 543 million euros on March 4.

It also pocketed 3.2 billion euros from the sale of its French operations to Macif's Aéma Groupe in February and an additional 122 million pounds from flipping its 40% stake in a joint venture in Turkey to Ageas Insurance International. ($1 = 0.8496 euros) (Reporting by Pamela Barbaglia, Stephen Jewkes, Carolyn Cohn and Alex Huebner; Editing by Leslie Adler and Richard Pullin)