MELBOURNE, Aug 21 (Reuters) – Top global miner BHP Billiton is a little nervous about the near-term outlook for commodity prices, its chief executive said on Tuesday. “We’ve said … that we’re a little bit more apprehensive about the short-term, but longer term, we still see a lot of strong demand for our products,” Chief Executive Andrew Mackenzie told reporters after the company reported a 33 percent rise in annual profit. “And in the meantime we continue to work our productivity so that we can retain margins even in the face of maybe some input inflation and possibly lower prices.”