The European Commission said the companies have yet to provide an important piece of information.

"Once the missing information is supplied by the parties, the clock is re-started and the deadline for the Commission's decision is then adjusted accordingly," Commission spokesman Ricardo Cardoso said in an email.

To address competition concerns, Halliburton proposed a new set of divestitures in January to regulators, with plans to sell assets of Halliburton and Baker Hughes with combined 2013 revenue of $5.2 billion.

However, Halliburton has yet to make a formal divestiture offer to the European Commission.

Once the formal offer is submitted, the commission will commence its due diligence process on the package, Iberia Capital Partners analyst Robert MacKenzie wrote in a note to clients on Monday.

MacKenzie recently lowered the probability estimate of the deal going through to 40-50 percent, due to the lengthy regulatory process.

The EU competition authority is concerned that the deal may reduce competition and innovation in more than 30 product markets, both onshore and offshore.

(Reporting by Foo Yun Chee in Brussels and Amrutha Gayathri in Bengaluru, editing by Louise Heavens and Maju Samuel)