On the anniversary of major ethnic violence, the top communist official of China's ethnic Uighur region vowed to strike down terrorists and separatists with "iron fists."

Zhang Chunxian, secretary of the Xinjiang committee of the Communist Party of China, said the situation in the northwest Xinjiang Uighur autonomous region is stable.

But the area faces "severe challenges," Zhang said.

"We should leave terrorists no place to hide."

Zhang was overseeing a counter-terrorism drill staged by special forces in the regional capital Urumqi to mark the anniversary of the July 2009 riots, a report by China's national news agency Xinhua said.

The government blamed overseas groups for inciting the riots which killed nearly 200 people, the Xinhua report said.

Zhang's speech comes after a failed plane hijack attempt by a group of people pretending to be disabled and on crutches.

Six passengers tried to hijack a Tianjin Airline plane minutes after it took off from Hotan Airport late last month, Xinhua said.

Police and witnesses said the hijackers dismantled an aluminum crutch into individual pipes and used them as weapons as they stormed the cockpit.

On-board security staff and flight attendants managed to overcome the assailants and the aircraft landed safely.

A spokesman for Xinjiang Airport Group, the Uighur regional airport operator, said security at its 16 airports has been beefed up.

In particular, the disabled should present hospital-issued disability certificates if they want to bring crutches or other mobility aids on board the plane, the Xinhua report said.

Passengers at Kashgar Airport, in southern Xinjiang near Hotan, must check their crutches and wheelchairs in as baggage.

Beijing has remained vigilant for the past three years against demonstrations and other public incidents in the restive region that could develop into clashes with police and get out of hand.

Around 8 million Turkic-speaking, mostly Muslim Uighur live in Xinjiang, which borders Mongolia and former Soviet republics.

Many Uighur say they are unhappy about the large influx of Han Chinese settlers, whom the Uighurs say increasingly marginalize their interests and culture.

During unrest in February, Xinhua reported that "a few rioters" armed with knives attacked "victims" in Yecheng county in Kashgar prefecture, killing 10 people.

Police reportedly killed "two assailants."

Radio Free Asia said at the time it received an e-mail message from an unnamed Uighur saying the clashes started in a local market by three Han Chinese men who insulted a Uighur youth.

A group of people aged around 18 years attacked the three Han Chinese, resulting in their death, the message said.

Chinese authorities continue to detain Uighurs suspected of participating in the 2009 riots, Amnesty International said in a written statement this week.

"Three years on, the (Chinese) government is still silencing people who speak out about July 2009," Catherine Baber, Amnesty International's Asia-Pacific director, said.

"The general trend toward repression that we see all over China is particularly pronounced in the XUAR," she said.

"Chinese authorities must reveal the whereabouts of those individuals subject to enforced disappearance and end the persecution of their family members seeking answers."

Amnesty said new testimony reveals "dozens, if not hundreds, of the Uighur ethnic minority, many of whom were arrested in the wake of the riots, are still disappeared."

The government continues to intimidate families seeking information about their disappeared relatives who revealed human rights abuses during and after the protests.

Of 20 Uighurs forcibly returned to China from Cambodia in December 2009 in connection with the July 5 riots, five reportedly were given life sentences.

Eight people are believed to have been given prison terms from 16-20 years, Amnesty said.