Reasons behind resignation of Karim Massimov, former Soviet nation’s longest serving prime minister, are unclear.

The prime minister of Kazakhstan has resigned after leading the oil-rich Central Asian nation for five years and shepherding its economy through the global financial crisis.

Karim Massimov’s resignation was accepted on Monday by a presidential decree, which also resulted in his appointment to the position of head of President Nursultan Nazarbayev’s administration.

Massimov was the longest-serving head of government in the former Soviet nation’s history and is widely respected in the international investor community.

Real power in Kazakhstan is held by Nazarbayev, but Massimov has been seen as an influential and competent steward of economic affairs.

His resignation had been expected, and is widely seen as an attempt by Nazarbayev to consolidate power.

A government source told the AFP news agency that Massimov’s place will be taken by Serik Akhmetov, first deputy prime minister.