Chinese soldiers in riot gear stand and sit at the main city square in the centre of Urumqi in China's Xinjiang region on Thursday. (David Gray/Reuters)

Military helicopters, armoured vehicles and trucks of police officers ramped up the propaganda campaign to maintain order in the capital of China's Xinjiang province on Thursday.

Many of the soldiers sent to quell the violence in Urumqi in the past few days have left, but security forces are maintaining a firm grip on the region. At least 156 people have died in ethnic violence between Han Chinese and minority Muslim Uighurs in the northwestern province of Xinjiang since Sunday.

Trucks covered in banners reading "We must defeat the terrorists" and "Oppose ethnic separatism and hatred" rolled through Urumqi as helicopters flew overhead, The Associated Press reported.

Armoured vehicles accompanied the trucks as they passed through the Uighur neighbourhood of Saimachang, according to Reuters.

Red stickers reading "Don't listen to any rumours" and "Keep calm and maintain public order" were placed outside apartment complexes.

Must attack 'hard-core elements'

The Uighurs say security forces gunned down many of Sunday's protesters. Officials have yet to give an ethnic breakdown of those killed.

Chinese army troops stand by as others on a truck are driven off outside a main mosque in Urumqi on Thursday. (Nir Elias/Reuters)

Following a meeting of China's Politburo, communist leaders called for stability and vowed to pursue punishment to violent rioters.

"We must by law severely attack those hard-core elements who planned and organized this incident and seriously violent criminals," the Politburo said.

It also called for "preventive measures" against "enemy forces who would undermine ethnic unity," and stressed the need to preserve social stability.

Li Zhi, Urumqi Communist party chief, told reporters Wednesday the death penalty will be sought for anyone found to be behind the riot's killings.

The protests started in Urumqi on Sunday, when demonstrators gathered to demand justice for two Uighurs killed in June during a fight with their Han co-workers at a factory in southern China.

1,000 injured, 1,400 detained

The protests turned into the deadliest ethnic unrest in the region in decades. Officials have said that more than 1,000 have been injured and about 1,400 have been detained.

Hundreds of vehicles, stores and street vendor stalls were also damaged or set ablaze during the protests, officials said.

In response to the riot, hundreds of Han Chinese rampaged through the city Tuesday with sticks and meat cleavers, looking for Uighurs and revenge.

The Uighurs, an ethnically Turkic, predominantly Muslim group, make up the majority in Xinjiang.

Their relations have often been tense, with the ethnic Han Chinese who predominate in the country. Many Uighurs feel they're discriminated against by the government in Beijing and a Uighur separatist movement has existed for decades.

With files from The Associated Press