President Vladimir Putin has postponed a visit to Kazakhstan, a spokesman for the Kazakh president said on Wednesday, and a Kazakh government source said Putin was unwell.
"The meeting in Astana has been postponed," Dauren Abayev, adviser and spokesman for Kazakh President Nursultan Nazarbayev said.
He declined to give a reason for the delay to the meeting planned between the allies for this week, or say when it would now take place.
A Kazakh government source said earlier that Putin's visit had been put off possibly due to illness.
"It looks like he (Putin) has fallen ill," he said.
The Kremlin was not immediately available for comment.
The president of Belarus, Alexander Lukashenko, had also been expected to join Putin and Nazarbayev for talks in Astana to discuss issues including economic cooperation and the conflict in Ukraine.
The Kazakh presidential spokesman said Lukashenko would travel to Kazakhstan at a later date.
Nazarbayev said on Wednesday he would seek re-election in a snap poll next month, which will almost certainly see him extend his 26 years at the helm of the oil-exporting nation.