Leaders of some of the EU countries which back David Cameron's calls for reform of the 28-nation bloc have said they are yet to see details of Britain's proposals for change.
Finnish prime minister Juha Sipila said he had not yet seen "concrete proposals", while Estonian PM Taavi Roivas said he was not expecting discussions to start until December.
They were speaking at the Northern Future Forum summit in Icelandic capital Reykjavik, where Mr Cameron is meeting leaders of northern European countries, including both EU members and non-members.
He has warned that it would not be beneficial for Britain to follow the model of countries like Norway, which remain outside the EU but are required to make payments to Brussels and comply with EU regulations in return for access to the single market.
Technical talks on Britain's proposals for EU reform have been under way with Brussels officials since June, and Mr Cameron has said he will provide details of his proposals to fellow EU leaders in a letter to European Council president Donald Tusk early in November, ahead of substantive discussions between national leaders at a Brussels summit in December.
Asked if he had seen concrete proposals yet, Mr Sipila told the BBC: "No, I haven't seen that."
And he added: "I fully understand the situation and the worries that you have on this issue. When you have concrete proposals, I think that Finland and other Nordic countries are very open to find a solution for that."
Mr Roivas said: "We are ready to open the discussions in December. We have discussed what the directions are, and there we have seen mutual interest as well. For us, it's very important."
Former deputy prime minister Nick Clegg said he was unable to comment on the progress of negotiations, but said it was important that Mr Cameron concludes them early enough to give him time to make the case for continued EU membership to the public before the issue is put to a vote in the referendum promised by the end of 2017.
Mr Clegg told BBC Radio 4's Today programme: "I think it is very important that the Government as a whole has the time and the space to make the bigger argument to the British people, once it has got beyond the nitty gritty of the renegotiation.
"The fundamental choice we have before us - Do we remain part of the world's largest borderless single market or not? Do we retain the clout of being part of that club or do we become more isolated? - is a very fundamental question about the future of our country which is there almost regardless of some of the terms of the renegotiation itself."
Mr Clegg said that all of the models for relationships with the EU offered by non-member European countries like Norway, Iceland, Liechtenstein or Switzerland were "worse than being a full member of the single market".
"All of them involve a significant loss of control in which you have to abide by the rules of the EU but you have no right to write those rules, and furthermore you have to pay your dues, you have to pay money into the EU coffers," said the former Liberal Democrat leader.
Kate Hoey, co-chair of the Labour Leave campaign for British withdrawal, told Today: "I think Nick Clegg - and indeed, I'm sorry to say, the Prime Minister - seems to be trying to introduce an element of fear into this whole debate.
"Are we really saying that Germany and other EU countries who rely on selling things to us are going to stop selling things to us?
"We are big enough and strong enough to be able to negotiate those terms without having to be under the protection and under the law of the EU.
"It's really sad in a way that our leaders in this country are not prepared to be standing up for our country internationally and recognising that we could do international deals which would be just as important as what we are doing at the moment, but we would also be able to get our own place back as a country on some of the international bodies."
The Labour MP for Vauxhall added: "The reality is that no-one on the sceptic side has said that they want the Norwegian option. We are all individual countries and we've all got our own interests."