Italian payments groups Nexi and SIA are exchanging confidential information as they explore a possible tie up to create an Italian payments powerhouse, three sources familiar with the matter said on Thursday.

Discussions, which have been dragging since last year, may accelerate in June when valuation will become the main focus, one of the sources said, adding that establishing a fair value for unlisted SIA may continue to prove a stumbling block.

The source said another element creating uncertainty over SIA's value was a key contract it has in place with top bank UniCredit, which can be ended after 2021.

Nexi is working with Bank of America and Mediobanca, while SIA has hired JPMorgan and Rothschild to advise on possible options which include also a stock market listing, two of the sources said. Nexi and SIA declined to comment.

(Reporting by Pamela Barbaglia in London and Valentina Za and Elisa Anzolin in Milan; editing by Agnieszka Flak)