China on Tuesday ordered tighter security checks at airports in its restive Xinjiang and Tibet regions, plus more than a dozen other cities, in the run-up to next month's Olympic Games.

From July 20, everyone arriving at an airport terminal building in the designated cities and regions should undergo a security check at the entrance "to guarantee civil aviation transport security during the Beijing Olympic Games", the Civil Aviation Administration of China (CAAC) said in a notice posted on its website.

CAAC said the "special security inspections" applied to Beijing and five other cities that will host Olympic events, plus airports in Xinjiang, Tibet and 10 other cities near the cities hosting Olympic events.

Airport security staff would "handle strictly according to the law" anyone found carrying flammable, explosive or other banned material, the notice said.

Xinjiang and Tibet appear to have been included in the list because of recent unrest in the two regions, where the Chinese government has reported that independence activists have conducted or planned terrorist activities.

CAAC has already required all Chinese airports to prevent passengers from carrying on board planes most liquids, lighters and matches, and has ordered security inspections of passengers' shoes.

The government said in April that it had foiled plots by two terrorist groups led by Uighurs in Xinjiang, including a plan to kidnap foreigners and carry out suicide bombings during the Olympics.

It also accused some Tibetans of terrorist attacks following widespread anti-Chinese protests and rioting in March and April.

But Uighur and Tibetan exile groups have accused China of using the terrorism claims as an excuse for a broad crackdown on dissent in Xinjiang and Tibet in the run-up to the Olympics.

Zhou Yongkang, China's top security official, said late last year that he was confident about security for the Olympics but saw remaining challenges from terrorism, separatism and religious extremism, which the government links as the "three evil forces" in Xinjiang.

"The Olympic Games is a big target for terrorism," state media quoted Zhou as saying.