"There are a series of national and regional approvals still pending after that," Deutsche Boerse CEO Carsten Kengeter said at an economic conference in reference to an investigation by European antitrust regulators.

"So it is still not clear at all that everything will work out," Kengeter added.

Concerned the $28 billion (£22.2 billion) tie-up could hinder competition in key financial market activities, the EU has opened a full investigation into the deal and the European Commission is expected to detail its objections in the second week of December.

Both companies can appeal against these or make concessions.

A thumbs up from the EU would then require the local watchdog in Germany's state of Hesse, where Deutsche Boerse is based, to approve the merger. Sources had previously told Reuters the regulator was concerned over a loss of power if Deutsche Boerse became part of the larger London-based group.

The owner of the Frankfurt stock exchange is obliged by law to support the development of Frankfurt as a centre for securities trading and Hesse's market watchdog has yet to give its view on the planned merger.

Asked about talk he could step down if the merger failed, Kengeter said only, "Deutsche Boerse is a wonderful company with and without me."

(Reporting by Andreas Kroener; Writing by Tina Bellon; Editing by Alexandra Hudson)