Oil prices globally have dropped by almost half since last summer, negatively impacting the economies of oil-rich countries, including Russia and Kazakhstan.
"Unfortunately, our union is under great stress because the low export prices on oil in both Russia and Kazakhstan, but we knew about these risks and the price disproportion, and there were worries," Nazarbayev said during a meeting with Russian President Vladimir Putin in the capital city of Astana.
The two presidents are also due to meet their Belarusian counterpart Alexander Lukashenko after the talks, originally scheduled for March 13, were postponed last week.
Deeper trade cooperation, global economic trends and the almost year-long armed conflict in eastern Ukraine are among key issues on the agenda of the summit, according to earlier press releases.
The EEU is an economic integration comprising five former Soviet republics of Russia, Belarus, Kazakhstan and Armenia, established on January 1, 2015. The union aims to achieve the free flow of goods, services, capital and labor across its member countries.
An accession by the central Asian republic of Kyrgyzstan to the EEU is pending.