Japanese trading house Mitsui & Co Ltd (8031.T) said it had sold its 25 percent stake in Silver Bell Mining to a unit of Grupo Mexico (GMEXICOB.MX) that already owns the remaining 75 percent in the operator of Silver Bell copper mine in Arizona.

The sale comes as part of Mitsui's three-year business plan, mapped out in May, to sell off 700-900 billion yen ($6.4-8.2 billion) of assets by March 2017 to fund investment plans and seek returns to shareholders.

"After examining our global assets, we've decided to sell some assets which are not among our top priority list," a Mitsui spokesman said on Thursday.

The firm, which did not disclose the deal value, plans to book a one-time profit of about 5 billion yen for the July-September quarter from the sale of stake in the copper mine that produces 20,000 tonnes of copper cathode a year.

This gain has been discounted into Mitsui's earnings outlook for the business year to March 2015, the spokesman said.

The Mexican miner's unit, AR Silver Bell, will become the sole owner of Silver Bell Mining following the deal.

After going on a spending spree during the global commodities boom, many Japanese trading houses have stretched their balance sheets and are now looking to sell off some of the assets to fund future investment plans.