Fortescue Metals Group has warned equity markets it is always willing to seize on weakness in its share price, saying that bearish markets were a factor behind this month's surprise $500 million share buyback.

Fortescue shares have halved over the past 20 months and fallen 33 per cent since February on the back of widening price penalties on its iron ore, and that trend prompted the miner to recently break its tradition of only announcing shareholder returns at its February and August financial results.