Sechin made the announcement during an April 30 meeting with European Energy Commissioner Andris Piebalgs, Itar-Tass reports.
Sechin said 19.5 billion cubic meters of gas is the ideal amount needed to keep Ukraine's gas pipeline system running and able to deliver gas to the European Union.
"In order to maintain uninterrupted transit of Russian gas to the European consumers, Ukraine must purchase and pump into underground storages no less than 19.5 billion cubic meters of gas in the near future," Sechin said.
If Ukraine fails to purchase gas as it has been, a repeat of last winter is likely, he said.
In January, a dispute between Ukraine and its Russian natural gas supplier caused shortages across Europe.