Exchange operators NYSE Euronext and Deutsche Boerse on Tuesday offered more concessions to assuage European authorities' antitrust concerns over their proposed $9 billion (5.8 billion pound) merger.

The parties have strengthened their original proposal with respect to European single equity derivatives by increasing the assets to be included in the divestiture, and to provide the purchaser of that business with an option to access Eurex Clearing for single equity derivatives products, the companies said.

In November, Deutsche Boerse and NYSE offered a first batch of concessions, proposing to sell significantly overlapping parts of their single-stock equity derivatives businesses in key markets, and give new, innovative rival products access to Deutsche Boerse's Eurex derivatives clearing house.

On Tuesday, the two companies went further and said they were also ready to offer rivals more extensive access to their clearing house for trading in innovative equity index and interest rate derivatives.

In addition, they said they would license the Eurex trading system to a third party interested in launching interest rate derivatives.

The European Commission is set to complete its review by February 9. The companies said they expected the deal to close after that, in early 2012.

People familiar with Deutsche Boerse's thinking told Reuters last week that the two aspiring merger partners could spin off parts of their derivatives arms to create a third-party competitor as a way to allay anti-trust concerns.

EU regulators met the exchanges on Tuesday last week and discussed whether rivals and users felt their proposals addressed antitrust issues with the derivatives market remaining the main concern, a person familiar with the talks said.

European competition authorities have expressed concern that the combination of Deutsche Boerse's Eurex and NYSE Euronext's Liffe will give the merged entity a monopoly over European listed derivatives trading.

The project is also facing close scrutiny from German authorities. A regional regulator said on Monday it had raised some objections to the exchange operators and was awaiting a reply.

This year, Eurex and Liffe have accounted for 97 percent of European stock futures trading and 93.7 percent of stock options trading, Federation of European Securities Exchanges data shows.

The exchanges have argued their market share is much lower if the vast over-the-counter (OTC) market is included in the assessment of the derivatives market.

But their merger plans were dealt a blow when the European Union decided to exclude OTC derivatives from its review of the deal.

(Reporting by James Regan and Ludwig Burger; Editing by Erica Billingham)