Halliburton and Baker Hughes said they've extended the timing of the earliest close of their merger by three weeks to Dec. 15.

The companies in a joint statement said they're amending their timing agreement with an anti-trust division with the Department of Justice from Nov. 25.

"Timing agreements are often entered into in connection with large, complex transactions, and provide the DOJ additional time to review responses to its second requests," the companies explained. "In light of the timing agreement, Halliburton and Baker Hughes have agreed to extend the time period for closing of the acquisition pursuant to the Merger Agreement to no later than Dec. 16, 2015."

Last year, rival Halliburton made a move to acquire Baker Hughes. Baker Hughes Chairman and Chief Executive Officer Martin Craighead said the merger would be "efficient."

Both companies have struggled to cope with the weak trajectory for crude prices, down about 50 percent from when the merger plans were announced in late 2014. Each has eliminated staff and each has reported poor results for the second quarter of 2015.

Rival oilfield services company Weatherford International described the current oil market as very challenging.

Halliburton and Baker Hughes said they were unloading several business units as they prepare for their eventual $35 billion merger. The combined units were valued in 2013 at $5.2 billion.

"Halliburton anticipates that the companies will complete the sales of these businesses in the same timeframe as, and the closing of the divestitures would be conditioned on, the closing of the pending Baker Hughes acquisition," it said in a statement.