European Union leader Donald Tusk is casting doubt over whether the EU can meet Britain's demands for staying in the 28-nation bloc.

Tusk said Thursday that Britain's "requests are tough."

British Prime Minister David Cameron has called a referendum on Britain's EU membership before the end of 2017.

The aim is to reach agreement with the EU on economic reform, cutting red tape and other changes by the end of this year.

But Tusk said "there is no guarantee that we can do this by December."

He added that the process of negotiating with London "is a very, very tough game."