With U.S. government pressure on the merger confirmed, oilfield services companies Baker Hughes and Halliburton said complaints were counterproductive.

Rumors surfaced earlier this week the U.S. Justice Department would follow its European counterparts in challenging the proposed merger of the two companies because of concerns about market competition. The two companies are the No. 2 and No. 3 in terms of market share, behind Schlumberger.

Bill Baer, the U.S. assistant attorney general, said the planned merger would consolidate the services sector into the hands of only a very few companies.

"This transaction is unprecedented in the breadth and scope of competitive overlaps and antitrust issues it presents," he said in a statement.

Halliburton and Baker Hughes unveiled plans to join forces in early 2015 as lower crude oil prices started to spill over into the economics of the upstream, or exploration and production, side of the energy sector. In November, Halliburton said it was offering $7.5 billion in senior notes to be issued in five tranches, with a maximum 5 percent fixed rate for a 30-year note maturing Nov. 15, 2045, to help fund the acquisition.

The companies in a joint statement noted that Halliburton "early in the process" proposed a divestment package to the Department of Justice "worth billions of dollars" that was meant to address specific competition concerns raised by U.S. federal authorities. On the federal decision, the companies vowed to launch a challenge.

"The companies believe that the Department of Justice has reached the wrong conclusion in its assessment of the transaction and that its action is counterproductive, especially in the context of the challenges the U.S. and global energy industry are currently experiencing," they said in a joint statement.

Their industry peers have joined forces in an effort to streamline operations in a market redefined by historically low crude oil prices, down about 60 percent from the highs above $100 that were commonplace in 2014.

In February, British energy company BG Group was fully incorporated into Royal Dutch Shell, which at $7 billion was the largest merger of its kind since Exxon and Mobil joined forces in the 1990s. This week, Schlumberger, the world's largest oil field services company, closed on its merger with its smaller industry counterpart Cameron International Corp.

In expressing its concerns earlier this year, the European Commission found only Baker Hughes, Halliburton and Schlumberger were able to provide necessary services to the industry, adding the merger of the former two would grossly inhibit competition from smaller potential market players.

Baker Hughes and Halliburton said they have until April 30 to decide to either continue their pursuit of regulatory approval or terminate the merger agreement, provided the judicial review extends beyond that date.