Two Hong Kong airlines said Friday that they received a warning from Taiwan about a possible bomb attack targeting a flight to the southern Chinese city.

Details on the nature of the threat were scant but the alert comes at a time of heightened tension on the Chinese mainland as Beijing responds to a spate of suicide bombings and stabbings by suspected Uighur separatists.

The warning also came the same week tens of thousands of people rallied in Hong Kong to remember the dead on the 25th anniversary of the Tiananmen Square crackdown, the only major commemoration that takes place in China.

The South China Morning Post, quoting an unnamed Taiwanese law enforcement officer, said a female carrying a bomb planned to fly from mainland China to Hong Kong on either Friday or Saturday, through a flight operated by Cathay Pacific or its Dragonair unit.

A spokeswoman for the city's flagship airline confirmed with AFP a threat message originating from Taiwan had been made known to the company.

"We are aware of a threat message with reference to our flights from mainland China to Hong Kong as shared by the Taiwan authorities," the Cathay spokeswoman said in an emailed statement.

"We will continue to work closely with the relevant authorities and have reminded our frontline teams to remain vigilant as usual," she added.

A second carrier, Hong Kong Airlines, also said it had received the same intelligence.

"Hong Kong Airlines believe there is a need to strengthen the level of internal security alert to ensure safety. The company has urged frontline officers to stay particularly alert," a spokeswoman told AFP.

Police in the Asian financial hub, however, played down the threat saying "no concrete intelligence" existed about an imminent terrorist attack.

In Taipei, an official with Taiwan's Aviation Police Bureau told AFP they had been informed by their "supervisors" about a potential threat. But the official declined to give further information on which agency had issued the warning.

Taiwan's Central News Agency reported that the intelligence came from the National Security Bureau on June 4, adding that the bureau notified authorities in Hong Kong and China.

Hong Kong's airport is one of the busiest in the world but reporting of terror plots against the city is rare.

Mainland China has been rocked by a series of recent attacks blamed on Uighur separatists from restive Xinjiang region.

Last month Beijing vowed a year-long campaign against terrorism, after attackers in the regional capital Urumqi killed 39 people in a suicide raid on a marketplace.

Beijing blames the attacks on organised terrorist groups seeking independence for Xinjiang.

Critics say China exaggerates the terror threat to justify hard-line measures, and point to economic inequality and cultural and religious repression of Uighurs as causes of unrest.

Mainland Chinese authorities were not immediately available for comment on the latest threat warning.