By Yuka Obayashi and Yoshiyasu Shida

TOKYO (Reuters) - Japan's Mitsui & Co plans to invest in natural resource assets, taking advantage of weak commodity prices, with an aim to more than triple its annual profit from the energy and metals segments to around $2 billion by 2020, its chief executive said.

Mitsui's move is in contrast to that of rival Mitsubishi Corp's, which has said it will freeze growth in its natural resources assets on a net basis for the next three years to ride out the commodities slump that forced it to record its first ever annual loss since its founding in 1954.

Mitsui also flipped into the red for the first time in its almost 70-year history, caught flat-footed like other global commodity players by the price rout brought on by weaker demand at top consumer China. But its strategy will be to "reinforce (its) areas of strength", Mitsui's CEO and president, Tatsuo Yasunaga, told Reuters in an interview on Friday.

The energy and metals segments have been among the biggest earners for Mitsui, with nearly all its record-high profits in the financial year to March 2012 coming from these two sectors.

"We'll be very selective ... But we won't be just waiting for a recovery in commodities markets. We'll be investing in the stakes which only become available to us because of the current conditions," Yasunaga said.

Earlier this week, Mitsui and Australia's Woodside Petroleum Ltd said they would invest $1.9 billion to develop the Greater Enfield reserves, a group of oil fields off Western Australia, given a sharp drop in costs made possible by a plunge in oil prices.

Oil futures are now just below $50 per barrel, off more than 12-year lows plumbed earlier this year but still significantly below their 2014 peaks of above $100.

However, the 55-year-old Yasunaga, who took Mitsui's top job in April 2015, stressed the need to also strengthen the firm's non-resource operations, or the areas that are not sensitive to volatile commodities markets and bring stable profits such as infrastructure and healthcare.

"It will be challenging, but our aim is to boost profits in non-resource segments to 200 billion yen ($1.95 billion) in 2020 from a planned 140 billion yen this year, while bolstering profits in resource operations to 200 billion yen from an estimated 60 billion yen," he said.

(Reporting by Yuka Obayashi and Yoshiyasu Shida; Editing by Himani Sarkar)