China's civil aviation authority has confirmed military exercises are behind widespread flight delays and cancellations this month, following days of rumours.

The Civil Aviation Administration of China (CAAC) said in a statement that "routine" military drills had impacted flights in the latter part of July, though it also cited weather and unspecified other reasons.

Cities affected included Shanghai, Nanjing and Qingdao in the east, and Wuhan and Zhengzhou in central China, the statement said.

Hundreds of flights were delayed at Shanghai's two airports, Pudong and Hongqiao, this week, according to local media, with around 170 delayed on Thursday alone according to the Shanghai Daily newspaper.

China's military controls much of the country's airspace -- some estimates say as little as 20 percent is reserved for civil flights. Industry officials say that results in longer flight times, less efficient routes and unaccountable delays.

China's capital Beijing was the worst city in the world for delays last year with only 18 percent of flights on time, followed immediately by Shanghai, according to the website www.flightstats.com.

China is holding military exercises across several provinces from July to October, state media have previously reported. China's defence ministry announced Thursday that troops had conducted a "missile interception test".

A report in the Beijing News newspaper said the flight delays would affect 12 airports mainly in eastern China from July 20 to August 15.

The CAAC is seeking to address the issue by setting up temporary air routes and allowing airlines to alter flight plans, said the statement posted on the CAAC website Thursday.

The administration had also banned applications for business jet flights, additional flights and charters at the affected airports, it said.