The European Commission has described some of the issues raised by UK 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.
'Is that it?' Eurosceptics turn on UK's Cameron after EU negotiation launch
Meanwhile, Eurosceptic members of Britain's Conservative Party turned on their party leader, Prime Minister David Cameron over his bid to renegotiate the country's ties with the European Union - with one calling his proposals "thin gruel."
After months of anticipation, Cameron published the four key areas in which he wants changes to the way the EU works, hoping to win over doubters who want Britain to vote to leave the bloc at a referendum due by the end of 2017.
"Is that it? Is that the sum total of the government's position in this renegotiation?" said visibly angry lawmaker, Bernard Jenkin, one of many dissatisfied Conservatives who stood up in parliament to voice discontent.
Cameron, who wants Britain to stay in a reformed EU, must not only persuade fellow EU leaders to give him what he wants, but also hold together a political party riven with decades-old disagreements on the merits of EU membership.
Vicious infighting over Europe led to the downfall of the last two Conservative prime ministers, Margaret Thatcher in 1990 and John Major in 1997. The EU referendum, if mishandled, is seen in some quarters as a threat to Cameron's term as prime minister.
Research published last month by the London-based Open Europe think tank, which lobbies for EU reform, found as many as one in five of Cameron's lawmakers is likely to vote to leave the EU.
At the heart of the dissent in parliament was the sceptics' belief that the government, having promised a fundamental reform, has not gone far enough to oppose what they see as the overreach of Brussels into domestic affairs.
"This is pretty thin gruel, much less than people had come to expect from the government," said parliamentarian Jacob Rees-Mogg. "It needs to do more. It needs to have a full list of powers that will be restored to the United Kingdom and to this parliament."
One lawmaker sarcastically offered Cameron his thanks for setting out a reform agenda that he viewed as not far reaching enough.
"That allows Eurosceptics to say 'no longer do we have to pretend there is going to be a substantial renegotiation'. We can get on and campaign to come out," lawmaker Peter Bone said.