Ukrainian President Petro Poroshenko has denied that behind the Ukrnafta case and Ihor Kolomoisky's stepdown as governor of Dnipropetrovsk region is a conflict in the upper echelons of authority.

"I wouldn't like it to look in any way as a conflict within the state system. I won't let any of this go ahead either as the president of the country or as guarantor of the Constitution. My current key position is the de-oligarchization of the country," Poroshenko said in a program on Saturday evening on ICTV television.

He said the government tries to put the country in order but that oligarchs try "to build chaos in the country."

He promised that the chief executives of all state companies, including Ukrnafta and Ukrtransnafta, would be selected through open competitions.

Poroshenko accepted Kolomoisky's resignation late on Tuesday.