Once Arch and Rio have complied with the request, Arch says the waiting period imposed by federal antitrust regulations will be extended by 30 days, “unless that period is extended voluntarily by the parties or terminated sooner by the FTC”.

"We believe the Jacobs Ranch acquisition is pro-competitive, and we are confident that it will create substantial value for customers and shareholders alike once completed," Arch chairman and chief executive officer Steven F Leer said.

With the sales and purchase agreement struck in March, the deal would give Arch some 381 million tons of coal reserves near its Black Thunder mine, plus associated infrastructure of a high-speed rail loadout, a recently added overland conveyor and a near-pit crushing system, along with a fleet of mining equipment and customer commitments.

In December, Rio announced plans to cut back 14,000 jobs and net debt by $US10 billion by the end of 2009.