"It would be wrong of me to speculate. Ukraine has emphasized that this will take some time," he told Canada's Maclean's.

"Some years ago, they [Ukraine] applied for membership, and I respected that. Then they decided to be a non-bloc country. Now they have started the process of reforming with the aim to apply for membership later on. If and when they apply, we will assess that application," he added.

Ukraine's Verkhovna Rada on December 23, 2014, amended Ukrainian laws to give up non-bloc status. The Ukrainian authorities believe that non-bloc status was unable to guarantee the country's security.