SAO PAULO (Reuters) -Talks for Brazilian firm Novonor, formerly Odebrecht, to sell a controlling stake in petrochemical company Braskem to a fund backed by businessman Nelson Tanure continue, despite the end of a 90-day exclusive negotiation deadline, Braskem said in a securities filing on Friday.

The exclusivity window has closed without both parties reaching an agreement that would exempt the fund of possibly paying for damages in a lawsuit that states the firm's salt mines have caused sinking ground in the city of Maceio and forced evacuations.

A source familiar with the Tanure-linked investment fund Petroquimica Verde told Reuters that no deal will be signed until they are certain that the environmental liabilities will not be transferred to the new partners and creditors.

Earlier this month Tanure said in a statement to Reuters that an agreement with the entities involved in the disaster was a "sine qua non (condition)" for the deal.

A second source close to the negotiations confirmed that resolving the environmental liability issue was from day one fundamental to advancing negotiations with Tanure.

Novonor's decision to continue negotiations with Tanure keeps the door open to a deal, but "sends an important signal to banks that were uncomfortable with the situation," the source added.

In the meantime, private equity firm IG4 Capital is working to launch a rival bid for Braskem's controlling stake as it aims to consolidate Novonor's bank debt and exchange it for Braskem shares, sources said.

Shares in Braskem were up some 2.4% in the afternoon trade, almost matching Brazil's benchmark stock index Bovespa, which gained around 2.5%.