EU leaders are expected to focus on migration, security, and economic issues as they wrap up their two-day summit in Brussels.
The summit, ending on June 23, follows a foiled suicide bombing in the Belgian capital this week and recent attacks in London and Paris.
It has also been overshadowed by discussions on Britain's drive to leave the 28-nation bloc.
On June 22, Prime Minister Theresa May unveiled British plans to ensure the rights of EU citizens in the country after Brexit.
European Commission President Jean-Claude Juncker said the proposals were "a step, but this step is not sufficient," dpa news agency reported.
German Chancellor Angela Merkel described the plan as "a good start," but added that there were "many other questions" about Britain’s departure from the EU.
British and EU negotiators officially started Brexit talks this week.
Britain voted to end its membership in a referendum a year ago and is set to leave the bloc by the end of March 2019.
Based on reporting by Reuters, AP, AFP, dpa, and the BBC