The pledge, reported by state media, was the first public comment by top leaders on the violence that has left 156 people dead in the city of Urumqi.
President Hu Jintao was forced to leave the G8 summit in Italy on Wednesday to attend to the crisis.
Thousands of troops remain on Urumqi’s streets to try to maintain order.
Tension has been high since the ethnic violence began on Sunday between Muslim Uighurs and Han Chinese.
State media Xinhua said Mr Hu had met other leaders at a Politburo meeting on Wednesday night to discuss the crisis in the north-western region.
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.
Reporters found few signs of the vigilante groups that had been roaming the streets since Sunday but tension remains high.
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”.
Many Uighurs remain angry over the arrests made since the violence began.
More than 1,400 people are thought to have been detained.
One woman told the Associated Press: “The men they arrested still have not returned. It has been three days and we haven’t been able to talk with them. We have no news.”
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.
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.