Kazakh authorities say they detained several people suspected of illegal oil sales during a large-scale security operation in the northwestern city of Aqtobe on December 7.
The National Security Committee (KNB) said that some of those detained are followers of the Salafi branch of Islam, which is officially banned in mostly Muslim Kazakhstan as extremist.
A KNB statement said that several containers of crude oil and oil products were impounded and that local companies were suspected of involvement.
The announcement comes days after a court in Aqtobe sentenced 29 men charged in connection with shooting attacks in the city in June that the government blamed on alleged Islamic militants.
Eighteen alleged attackers, five civilians, and three members of Kazakhstan's security forces were killed in that violence.
On November 28, seven of the 29 defendants were sentenced to life in prison, while the other 22 received prison terms ranging from two to 25 years. Investigators described all the defendants as Salafists or their supporters.
Critics of authoritarian President Nursultan Nazarbaev's government say the Kazakh authorities sometimes wrongly accuse certain groups of Muslims of extremism or blame extremists for crimes.
