While the Irish government wants to keep the entire region within the EU's customs union, Northern Ireland's Democratic Unionist Party said it wouldn't accept a post-Brexit deal that involved "internal barriers" with Britain.
He told Sky News's Sunday with Niall Paterson: "We have always had exceptions for Ireland - whether it's in our voting rights, our rights of residence in the United Kingdom, we have always accepted a certain asymmetry and that will have to be part of whatever agreement we come to with the European Union but we can't come to a final answer to the Irish question until we get an idea of the end state".
But Liam Fox, the International Trade Secretary, hit back, saying: "We don't want there to be a hard border, but the United Kingdom is going to be leaving the customs union and the single market".
Resolving the issue of Northern Ireland's border after leaving the European Union can not be completed until talks on a trade deal with Brussels have progressed, Britain's Secretary of State for International Trade Liam Fox says.
Speaking before the Oireachtas Committee on the Implementation of the Good Friday Agreement, he said we needed a lot more clarity that recognises the uniqueness and vulnerability of Northern Ireland and of the island of Ireland as a result of the UK's exit from the European Union.
Liam Fox has said a final decision on the Northern Irish border can not be made until a UK-EU trade deal has been agreed, despite warnings from Brussels that trade talks can not proceed unless an agreement is reached within days.
The tweet follows Tusks meeting with Theresa May at the EU's Eastern Partnership Summit, where May was given a deadline of 4 December to make finalize the Brexit Bill in order to move forward with the next phase of negotiations.
Ireland's European Commission member Phil Hogan raised the stakes yesterday, telling The Observer that he was amazed at the "blind faith that some in London place in theoretical future free-trade agreements". "These are solved by the single market, but not in an FTA".
Party leader Arlene Foster marked the issue as one of her red lines at the DUP party conference on Saturday.
Fox said that option was out of the question.
Theresa May told reporters that "There are still issues across the various matters that we are negotiating to be resolved".
"I don't think it means that the world has ended but I do think it's a setback", she said.