Beijing, meanwhile, again accused US-based Uighur leader-in-exile Rebiya Kadeer of organising the disorder.
Tension has been high since the ethnic violence began on Sunday between Muslim Uighurs and Han Chinese.
State-run Xinhua new agency said Mr Hu had met other leaders at a Politburo meeting on Wednesday night to discuss the crisis in the north-western region.
AT THE SCENE, 9 JULY
Quentin Sommerville, BBC News, Urumqi
A huge column of hundreds of paramilitary vehicles moved through Urumqi this morning, flying above them two helicopters, one dropping leaflets calling for calm and blaming the recent troubles on outside enemy forces.
In Han Chinese neighbourhoods people walked their dogs and practised Tai Chi but in the many Muslim Uighur areas of town, locals said they were still afraid to leave their homes.
The show of force here has brought calm, there was little reported violence overnight, but there is still deep underlying suspicion from both sides in this ethnically split city.
A statement issued on Thursday said that stability in Xinjiang was the "most important and pressing task".
"The planners of the incident, the organisers, key members and the serious violent criminals must be severely punished according to law," it said.
Local Communist authorities have already said that those found guilty of murder in the riots will be executed.
There remains a huge security presence in Urumqi as the authorities there try to restore normality.
On Thursday, helicopters dropped leaflets and trucks blared out messages, appealing for calm and blaming extremists for the violence.
The BBC's Quentin Sommerville, in Urumqi, says that in one of the Uighur neighbourhoods, mosques were open but life was not back to normal for the residents, who said they were scared of the security forces by day and of the Han Chinese by night.
Security forces continue to separate the Uighur and Han neighbourhoods.
But reporters found few signs of the vigilante groups that had been roaming the streets since Sunday.
One Han woman told the AFP news agency: "How can it return to normal with so many soldiers? I've counted 42 military trucks so far, and more trucks just came by."
Crowds of Han Chinese have been cheering on the security forces as they travelled in trucks carrying banners that declared "We must defeat the terrorists" and "Oppose ethnic separatism and hatred".
XINJIANG: ETHNIC UNREST
Main ethnic division: 45% Uighur, 40% Han Chinese
26 June: Mass factory brawl after dispute between Han Chinese and Uighurs in Guangdong, southern China, leaves two Uighurs dead
5 July: Uighur protest in Urumqi over the dispute turns violent, leaving 156 dead - most of them thought to be Han - and more than 1,000 hurt
7 July: Uighur women protest at arrests of menfolk. Han Chinese make armed counter-march
8 July: President Hu Jintao returns from G8 summit to tackle crisis
Xinhua also reported government sources had said there was evidence linking the World Uighur Congress, led by Rebiya Kadeer, to the riots.
It said that in the days beforehand, the group met, plotting to instigate the unrest using the internet and mobile phones, and that Mrs Kadeer had tipped off her brother in Urumqi on the eve of the disorder.
"All these facts pointed to Kadeer, whose personal experience bore further evidence of her splittist connection," the agency said.
The violence began on Sunday when Uighurs rallied to protest against a deadly brawl between Uighurs and Han several weeks ago in a toy factory in southern Guangdong province.
Officials say 156 people - mostly Han - died in Sunday's violence. Uighur groups say many more have died, claiming 90% of the dead were Uighurs.
More than 1,400 people are thought to have been detained.
There were further protests on Tuesday when Uighur women rallied against the arrest of family members.
Groups of Han Chinese armed with clubs then marched through the streets in a counter-protest that police broke up with tear gas.
Tensions have been growing in Xinjiang for many years, as Han migrants have poured into the region, where the Uighur minority is concentrated.
Many Uighurs feel economic growth has bypassed them and complain of discrimination and diminished opportunities.
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