"We all want a close and strong future relationship with the UK ...... But before discussing the future, we have to sort out our past," he said.
Tusk made the plea ahead of Saturday's EU summit where leaders are expected to adopt guidelines for Brexit negotiations.
The guidelines will define the framework for negotiations with the UK and set out the overall positions and principles that the EU will pursue during the talks.
Leaders of the remaining 27 EU members are meeting for the first time since British Prime Minister Theresa May announced in late March the start of the Brexit process.
XINHUA/MNA