Such a commitment should be included in any final declaration reached at the UN Oceans Conference (UNOC) in France in June, more than 100 environmental groups wrote in an open letter to governments.

Fossil fuels are the main contributor to climate change, which is altering the chemistry of oceans, driving record-high sea temperatures, and risking the survival of tropical coral reefs and the life and economies they support.

"If we truly want to protect the ocean, we must end the destructive practice of oil and gas exploration in marine environments immediately," Nicolas Entrup, from marine conservation group OceanCare, said in a statement.

"The third UN Ocean Conference must be a moment of action, not just of empty phrases."

Governments at UNOC should secure the adoption of a "state-level prohibition on all fossil fuel exploration activities" across the world's oceans and the "phase out of existing fossil fuel extraction", the letter said.

A first draft of the declaration already in circulation makes no mention at all of fossil fuels.

Such a ban would be difficult to secure, as decisions made at UNOC are by consensus and require broad approval, including from oil- and gas-producing nations.

Governments and businesses make conservation pledges at UN environment summits, but such declarations are voluntary and not legally binding.

At the UN COP28 climate summit in 2023, nearly 200 nations agreed to "transition away" from fossil fuels toward less polluting forms of energy.

But many countries -- such as the United States under President Donald Trump, and Brazil, which is hosting this year's COP30 climate conference -- are seeking to expand fossil fuel production.

Dozens of heads of state are expected to attend UNOC between June 9 and 13 in the city of Nice on the French Riviera, which is co-hosting the conference with Costa Rica.

Stronger rules for ocean governance, overfishing and deep-sea mining, as well as a global treaty on plastic pollution, are among issues on the agenda.