BEIJING: China vowed to strike with an "iron fist" at separatist forces in its restive Xinjiang region, as it rolled out a heavy security presence for the third anniversary on Thursday of deadly ethnic riots.

The pledge to crack down on "separatism, religious extremism and terrorism" came as Amnesty International condemned China for what it said was repression against ethnic Uighurs in Xinjiang who had spoken out against rights abuses.

Regional Communist Party chief Zhang Chunxian visited riot police in the Xinjiang capital of Urumqi on Wednesday ahead of the sensitive anniversary date and told them they must crush government opponents.

"Remain on high alert for every kind of hostile force and strike with an iron fist at the forces of separatism, religious extremism and terrorism," Zhang said, according to a statement on the regional government's website.

"Wherever they appear is where we must strike them down, violent terrorists must find nowhere to hide."

Security forces held "counter-terrorism" drills in Urumqi on Thursday, according to the state-run Xinhua news agency.

Xinjiang is home to around nine million Uighurs, a Turkic speaking, largely Muslim ethnic minority.

Many Uighurs accuse China's rulers of religious and political persecution, while complaining that their homeland is being inundated with the nation's dominant Han ethnic group.

China denies claims of repression, saying it has brought badly needed modernisation and economic development to the vast and landlocked region bordering Central Asia.

In some of the worst ethnic violence to hit China in decades, Uighurs began attacking Han in Urumqi on July 5, 2009, triggering days of clashes in which 200 people from both sides died, according to the government.

In the aftermath, police rounded up and jailed scores of participants, then executed at least nine people accused of instigating the riots.

In its statement to mark the anniversary of the riots, rights group Amnesty said dozens, if not hundreds, of Uighurs, were missing and authorities continued to detain people for expressing dissent.

"The general trend toward repression that we see all over China is particularly pronounced in Xinjiang," Amnesty's Asia-Pacific director, Catherine Baber, said in the statement.

Chinese authorities said the latest "terrorism" threat came last week when six Uighurs tried to hijack a commercial plane.

Passengers and crew overwhelmed the hijackers and the plane landed safely, according to the official accounts. AGENCIES