Although Yermak had captured the city of Isker and its environs, several Tatar tribes around him did not abstain from fighting with Kazaks on each opportunity that they found. Particularly, Küçüm Khan's brother, Muhammed Kul was involved in a severe struggle against Kazakh people. Muhammed Kul who had been famous with his heroism posed a great threat for Ytermak and it put the domination of Kazakhs in Isker into jeopardise day by day. It was a precondition to annihilate this Tatar leader at all in order to establish permanent dominion in Sibir. Muhammed Kul organised sudden attacks with his forces in his retinue and he retreated and disappeared with great speed. Therefore, it was very difficult to catch him. It has been known that there had not been any union among Tatar beys before the penetration of Kazakhs into Sibir. Upon the victory of Kazakhs, the enemies of Küçüm Khan and his supporters increased a lot; and treacheries arose. A treasonist among the princes named as Sinbahti sent a fellow to Yermak and informed the place where Muhammed Kul was. The Kazakh elected-chief immediately sent his soldiers to that place and he caught Muhammed Kul with a sudden attack.
The captivity of Muhammed Kul was a heavy shock for Küçüm Khan. Pursuant to this incident, several Tatar celebrities abandoned the Khan. The situation in the Sibir country got worse at all. Meanwhile, the former khan of Sibir, Seyyid Ak who was the son of Yadigar's brother, Bekbulat emerged with a claim for the khanate. One of karaca (one of the greatest princes) of Küçüm Khan immigrated to the banks of Tura River and he got separated from the Khan. Therefore, Sibir Tatars ran after their own personal benefits rather than fighting with cooperation against their common enemy under the most difficult circumstances of their history. They did not abstain from any treacheries, intrigues and wars among brothers in order to achieve their ambitions. Through these attitudes, they even facilitated the works of a few Russian Kazakhs that came to Sibir.