FRANKFURT/NEW DELHI, March 25 (Reuters) - Discussions of a possible sale of Thyssenkrupp's steel unit to Jindal Steel International could be called off due to differences over pension liabilities, investments and energy costs, four people familiar with the matter said.

While talks over a sale of Thyssenkrupp Steel Europe (TKSE) are ongoing and could still result in an agreement, a deal is now seen as less likely after nearly six months of due diligence and discussions, the people said.

The companies could decide to officially stop negotiations as soon as next month, one of the people said.

Thyssenkrupp has tried to sell TKSE several times in the past decades, pursuing everything from listings to spinoffs and joint ventures to outright sales of the cyclical high-cost business.

Failure to sell TKSE would mean a setback for the plan by Thyssenkrupp CEO Miguel Lopez to turn the storied German engineering group into a holding by divesting stakes in all of its business divisions ranging from car parts to clean-tech.

Among the factors complicating talks are 2.4 billion euros ($2.8 billion) of pension liabilities tied to TKSE, which has been a hurdle in past sales efforts, as well as differing ideas over how much future investment is needed, the people said.

In addition, there has been growing unease at Jindal Steel International over rising energy costs in Europe, the second source said. Energy costs in Europe were already higher than in the United States and Asia, and they have soared further as a result of the Iran war.

Thyssenkrupp said on Wednesday confidential talks with Jindal Steel International and labour representatives continued, adding that matters of valuation, obligations and future investments would need to be agreed between the parties.

Jindal Steel International, the international steel arm of the Naveen Jindal Group, had no immediate comment.

Earlier this month, Lopez said the group would continue with TKSE's restructuring "with or without Jindal," while Thyssenkrupp's deputy supervisory board chairman, Juergen Kerner, last week said talks had stalled.

Lopez has also said that planned EU measures to protect the bloc's underperforming steel sector had boosted investor sentiment and strengthened Thyssenkrupp's position in negotiations. 

Jindal Steel International in September made an indicative offer for TKSE that includes the completion of a green steel production site in Duisburg and a more than 2 billion-euro ($2.31 billion) commitment to establish additional electric arc furnace capacity.

($1 = 0.8622 euro)