EU leaders cast doubt over British Prime Minister David Cameron's controversial reform demands, calling them against the core principles of the 28-member bloc.

Cameron has said he would be "battling hard" through the night at the European summit in Brussels for his EU reform demands. That’s ahead of a referendum on the UK’s membership slated to be held by the end of 2017.

"We're not pushing for a deal tonight but we're pushing for real momentum so that we can get this deal done," Cameron told reporters upon his arrival to Brussels.

Greater protections for non-euro countries, exclusion for Britain from the “ever closer union" goal in the EU's treaties, greater economic competitiveness, and a limit to benefits for EU migrants are the Cameron is expected to tell his EU partners.

However, European Council President Donald Tusk has warned that some of Cameron’s proposals are “unacceptable”.

There is almost unanimous opposition in the EU to Cameron's call for a four-year limit before EU migrants working in Britain can claim benefit payments.

Earlier, European Commission Chief Jean-Claude Juncker said the bloc is seeking a fair deal over the future of Britain’s membership. He said the commission is ready to look for other options aside from the one proposed by the British premier.

French President Francois Hollande also pushed back against some of Cameron's demands, saying it is unacceptable for him to overturn the bloc's core principles.

"If it is legitimate to listen to the British prime minister, it is unacceptable to revise founding European commitments," Hollande told reporters ahead of summit in Brussels.