BILLINGS, Mont. (AP) — A Wyoming mining company has signed an option agreement allowing it to ship up to 16 million tons of coal a year through a planned West Coast port.

Cloud Peak Energy said Wednesday's deal involving Washington state's proposed Gateway Pacific Terminal will allow it to expand overseas sales amid weak domestic demand.

The company has mines in Wyoming and Montana. It shipped 4.4 million tons of coal to Asian customers in 2012.

Gateway Pacific is proposed by SSA Marine in the town of Cherry Point and could begin commercial operations by 2018.

Cloud Peak has a separate option agreement to ship 5 million tons of coal annually through a second Washington port, the Millennium Bulk Terminals in Longview.

Both port proposals are strongly opposed by conservation groups.