After negotiations going deep into the night with European Union leaders, President Donald Tusk emerged to say that much more work is needed to reach a deal that keeps Britain in the bloc.
"For now, I can only say that we have made some progress, but a lot needs to be done," Tusk told reporters in Brussels on February 19.
Spanish acting Prime Minister Mariano Rajoy said he remained optimistic about the chances of a deal -- possibly within the day.
But leaders of Italy and Ireland said they were less optimistic than they were going into the 15-hour day of meetings on Britain's reform demands and migration issues.
Tusk said he and European Commission President Jean-Claude Juncker would hold a series of bilateral meetings on February 19, starting with British Prime Minister David Cameron to discuss his reform demands.
After that, Tusk said he would meet with French President Francois Hollande, Belgian Prime Minister Charles Michel, and Czech Prime Minister Bohuslav Sobotka.
Hollande has reservations about Cameron's demands for safeguards for countries that do not use the euro currency, while Michel objects to calls to exclude Britain from the EU's goal of "ever closer union."
"I want Britain to stay in the EU. But I hope most of all that Europe can advance, can be stronger, and that no chief of state could stop that," Hollande said on arrival at the summit. "It's the European Union in question, not just one country."
The Czech premier meanwhile leads a group of four Eastern European countries that object to Cameron's request for a limit to welfare benefit payments for EU migrants working in Britain.
Cameron had warned that he might walk away from the summit if the deal is not good enough.
"If we can get a good deal, I will take that deal. But I will not take a deal that doesn't meet what we need," he said.
Also at the summit, leaders agreed to hold a special meeting with Turkish leaders on the migration crisis at the beginning of March, Tusk said.
Turkish Prime Minister Ahmet Davutoglu had been scheduled to join the summit, but cancelled his trip due to a major bomb attack in Ankara.