BRUSSELS (Reuters) - EU antitrust regulators will open a full-scale investigation into German industrial group Siemens' (>> Siemens AG) $7.6 billion (5 billion pound) bid for U.S. oilfield equipment maker Dresser-Rand Group Inc (>> Dresser-Rand Group Inc.), two people familiar with the matter said on Wednesday.

The move will ratchet up the pressure on the companies to offer concessions to allay the European Commission's concerns regarding the deal, one of Siemens' biggest acquisition.

The sources said neither company had so far offered anything during the Commission's preliminary review of the deal. They declined to provide details of the concerns.

Commission spokesman Ricardo Cardoso and Siemens declined to comment.

The EU competition authority is expected to announce it is launching an in-depth inquiry on Friday, which will automatically trigger a 90-working-day extension to its scrutiny.

Siemens is seeking to reinforce its presence in the U.S. shale oil and gas industry through the deal.

Siemens Chief Executive Joe Kaeser told journalists at the company’s quarterly results presentation last month that he expected to close the transaction this summer.

(Reporting by Foo Yun Chee; additional reporting by Georgina Prodhan in Frankfurt; editing by Philip Blenkinsop)

By Foo Yun Chee

Stocks treated in this article : Dresser-Rand Group Inc., Siemens AG