Major convenience store operator Ministop Co. started an experiment this week to charge for plastic bags at two of its stores in Chiba Prefecture amid heightened environmental awareness worldwide and the government's policy to ban free plastic bags.

The experiment began Monday at two stores next to the headquarters of Ministop's parent, Aeon Co., in the city of Chiba, and the company is considering expanding the practice to 40 stores by next February.

The stores will charge ¥3 per bag, 10 percent of which comes from plant-derived polyethylene.

Calls for an international treaty banning single-use plastic waste are growing amid worsening ocean pollution, a key issue during a meeting of the environment and energy ministers from the Group of 20 economies at Karuizawa, Nagano Prefecture, earlier this month.

The United Nations Environment Program estimates 300 million tons of plastic waste are disposed of worldwide every year, with 8 million tons ending up in the oceans.

Trade minister Hiroshige Seko said at the meeting that Japan will pursue having all stores charge customers for plastic bags, and that the government is aiming to have the policy take effect by April.

Seven & I Holdings Co. announced it will completely stop the use of plastic bags by 2030 with plans to shift to paper bags and other alternatives. Since April, Seven-Eleven Japan Co. has been conducting an experiment to allow customers in Yokohama to choose between plastic or paper bags.

Lawson Inc. plans to halve the number of plastic bags used by 2030 compared with 2017. FamilyMart Co. also seeks to reduce the weight of plastic bags to cut down on plastic consumption.