BRUSSELS (AP) — A European official says British Prime Minister David Cameron and European Council President Donald Tusk are meeting Sunday night to discuss the EU reforms Britain is asking for ahead of a referendum on continued membership.

Following an EU summit on Turkey and the refugee crisis on Sunday afternoon, Cameron and Tusk are holding discussions about which major stumbling blocks remain over the extent of such reforms.

The U.K. will hold a referendum on whether or not it should remain in the 28-nation EU by the end of 2017.

Cameron hopes to secure a looser relationship between Britain and the bloc. He hopes to get into detailed negotiations on a deal at a December summit focused on Britain's EU membership.

The EU official, who spoke on condition of anonymity due to the sensitivity of the matter, said both Tusk and Cameron know the substance of the reforms, and that the talks were "more about the process itself."

In recent months, Tusk has been speaking to other countries about their preferences for how such a reform would work. Several member countries had previously asked for detailed proposals so that negotiations about Britain's departure could begin.

The EU official said "serious issues" remained and that any agreement on reforms might not be reached in the immediate future.

This story has been corrected to say talks are on EU reforms not on Brexit.