BEIJING, April 14 (Reuters) - Chinese police have detained five people, possibly Tibetans, whose "suspicious remarks" prompted the return of their flight half an hour after take-off, local media reported on Monday.

China has tightened security of its airways ahead of the Beijing Olympics in August and says it foiled a plot last month by Muslim Uighur separatists to blow up a plane.

The flight on Sunday afternoon was from the southern city of Shenzhen and bound for Chengdu, capital of the southwestern province of Sichuan, the Southern Metropolis Daily said on Monday.

Deadly anti-Chinese riots hit Tibet and Tibetan-populated areas in neighbouring provinces, including Sichuan, last month. Many in-bound flights to the Himalayan region stop over in Chengdu.

The five passengers talked in a language "others could not understand", the newspaper said in a report on its Web site (www.nddaily.com).

One of them smiled at a flight attendant and said to her "You look very beautiful" and "Read the news tomorrow", it quoted a witness as saying.

"Perhaps it was this sentence that caused alarm among the crew," the witness was quoted as saying.

The plane landed back at Shenzhen where police took away the five passengers for investigation, the report said.

The caption of a photo accompanying the report said police checked the information on "the several Tibetans who made the remarks".

"Because of the many suspicious points in the case, Shenzhen's anti-terrorist office may intervene (into the probe)," the report said.

China says it foiled an attack on a flight to Beijing from Urumqi, capital city of the restive Muslim region of Xinjiang in the far west, on March 7.

The plane made an emergency landing after the crew found a 19-year-old ethnic Uighur woman with cans of flammable liquid in the toilet.

The government says the attack was plotted by Uighur separatists to disrupt the Olympics. (Reporting by Guo Shipeng; Editing by Nick Macfie)

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