The European Commission has described some of the issues raised by Prime Minister David Cameron to reform the EU as "highly problematic".
Some of Mr Cameron's proposals for talks to make sure that Britain stays in the EU may be feasible, Commission spokesman Margaritis Schinas said, adding that others ranged from "difficult to worse".
He specifically referred to proposals where freedom of movement would be limited by allowing the UK to restrict benefits for migrants from other member states.
Mr Schinas said: "Some things are highly problematic as they touch upon the fundamental freedoms of our internal market. Direct discrimination between EU citizens clearly falls into this last category."
German Chancellor Angela Merkel said she has been in touch with Mr Cameron over his demands for EU reforms, adding that he is bringing "no surprises to the table".
Ms Merkel told reporters in Berlin that she spoke with Mr Cameron by phone on Monday and talked about his proposals. She said that she is willing to work with him on them.
She said that "some are easier than others", but added that "if one has the spirit that we can solve these problems, then I'm convinced it can be done".
Meanwhile, the Czech prime minister rejected Mr Cameron's demand to include a limit on EU free movement.
In a statement, Bohuslav Sobotka said that any attempts to limit freedom of movement poses "a serious problem for the Czech Republic".
Mr Sobotka said that "the right to work and live anywhere in the EU is absolutely essential to us due to our historical experience".
He added that freedom of movement is considered by the Czechs to be a key advantage of EU membership and "it is impossible to imagine" his country giving it up.