Brexit negotiators are holding a round of 'technical' talks later, with Northern Ireland as one of the items on the agenda.
In December, the EU and UK agreed a deal that guaranteed no return to a hard border.
But EU chief negotiator Michel Barnier suggested the two sides have different interpretations of what was agreed.
The sticking point concerns a UK pledge to follow EU rules relating to cross-border co-operation.
The UK agreed that, in the absence of an overall deal, it will continue to fully align with the rules of the customs union and single market that are necessary for cross-border co-operation and the protection of the all-island economy.
In relation to alignment, Mr Barnier said the EU is "working with British authorities on the content of that sentence".
He added that, in his view, there were "a large number of rules where this coherence or alignment" would be needed.
However, UK ministers, including the Brexit Secretary, have suggested the areas requiring alignment would be minimal.
Irish Prime Minister Leo Varadkar said Dublin is seeking to have the commitments made in December written into the legally binding withdrawal agreement.
He said that is currently being worked on, describing it as "significant" that the latest talks are about the withdrawal agreement rather than a trade deal.
"We are not into talks about trade yet, because we think it's important that we prioritise the withdrawal agreement and transition phase and make sure what was agreed in December is written into the legal text of that agreement," he added.
Mr Varadkar was responding to criticism from Sinn Féin that he had been "naive" in light of the UK government's restatement of its policy to leave the customs union.
Meanwhile, the government's 11-strong Brexit cabinet sub-committee will meet on Wednesday and Thursday in an attempt to agree a vision of post-Brexit relations with the EU.
The outcome of that meeting is then expected to be shared with senior EU officials on Friday.