Beijing and Tianjin were among China's 10 most-polluted cities in June, with high levels of prime pollutants, such as fine particulate matter and ground-level ozone, leading experts to warn that the excessive level of ozone has become a major problem for the environmental authorities.

The cities ranked joint 65th in a list of 74 major cities in which air quality is monitored every month. While it was the first time since January last year that Tianjin has been ranked near the bottom of the list, Beijing has featured three times in the same period, according to a report published by the Ministry of Environmental Protection on Friday.

Days when air quality was good in the Chinese capital accounted for 36.7 percent of June, a 6.6 percent decline year-on-year. However, while levels of major airborne pollutants such as sulfur dioxide fell, the level of ozone rose, the report said.

Both cities saw excessive concentrations of ground-level ozone and PM2.5, fine particulate matter less than 2.5 microns in diameter that is hazardous to human health.

"The ranking doesn't mean pollution-control efforts are not working, but it indicates that further efforts are needed," Qiu Xiaowen, head of the Tianjin Environmental Monitoring Center, said on Friday.

He added that the excessive concentrations of PM2.5 and ozone are closely related and comprehensive measures will be required to reduce levels of airborne substances, such as nitrogen oxide and volatile organic compounds.

Ground-level ozone, which is harmful to health, is mainly generated through complex photochemical reactions in the atmosphere related to heat and strong sunshine, which means it is an acute problem during the summer months.

High levels of ozone resulted in a rise in the number of severely polluted days in the Beijing-Tianjin-Hebei cluster, where days with good air quality accounted for 48.3 percent of June, a year-on-year decline of 6.1 percent, the report said.

Zhang Yuanhang, a professor at Peking University's College of Environmental Science and Engineering, said excessive levels of ozone have been recorded in areas in the Yangtze and Pearl River deltas during summertime, and urged the government to implement more measures to tackle the problem.

He said the high levels of ozone levels highlighted inadequate controls of emissions of nitrogen oxide and volatile organic compounds, and suggested the two pollutants should be added to the targets outlined in the Action Plan for the Control and Prevention of Air Pollution.

Many cities have already imposed targeted measures. Qiu from the Tianjin center, said the authorities have strengthened monitoring of industries responsible for the discharge of volatile organic compounds, and Beijing has levied new taxes on them as a means of reducing emissions.