He said some key producers including Libya and Nigeria had significantly increased output in recent months, according to Reuters.
"In the current context we see the market rebalancing in the second half of the year. But if production increases in some of the key producers this may change the picture," Birol told Reuters on the sidelines of an Energy Institute event.
"Whatever OPEC do, if the prices go up, there will be a response from shale oil producers," he said.