Only about 4% of the world's oceans remain undamaged by human activity, according to the first detailed global map of human impacts on the seas.

A study in Science journal says climate change, fishing, pollution and other human factors have exacted a heavy toll on almost half of the marine waters.

Only remote icy areas near the poles are relatively pristine, but they face threats as ice sheets melt, it warns.

The authors say the data is a "wake-up call" to policymakers.