A new report shows China's busy ports have mostly failed to regulate heavy emissions of sulfur oxide and other pollutants from cargo ships and other port vehicles.

The U.S.-based National Resources Defense Council found in its report that marine activity makes up half of all sulfur oxide emitted in Hong Kong and about a third of all nitrous oxide. Similarly, such activity makes up about two-thirds of sulfur oxide emitted in the neighboring port of Shenzhen.

Only a few Chinese cities such as Hong Kong and Shenzhen have moved to regulate port emissions, while the national government is not enforcing emission standards. Seven of the world's 10 busiest container ports are in China.

Chinese officials have pledged to crack down on overall pollution plaguing many of the country's cities.