A joint UK and EU committee focused on how to implement the Northern Ireland part of the Brexit deal held its first meeting on Thursday.
The specialised committee is made up of government officials rather than politicians.
The UK delegation included senior Stormont official, Andrew McCormick.
The Brexit deal, which was agreed in October, treats Northern Ireland differently from other parts of the UK.
The EU said the meeting took place in "a constructive atmosphere".
The UK said that its approach "will be focused on protecting the Belfast/Good Friday Agreement and gains of the peace process, and on preserving Northern Ireland's place in the UK".
At the end of the transition period on 31 December, Northern Ireland will continue to follow EU rules on agricultural and manufactured goods, while the rest of the UK will not.
Additionally, the whole of the UK will leave the EU's customs union but Northern Ireland will continue to enforce the EU's customs code at its ports.
This will mean some new checks and processes for goods coming into Northern Ireland from other parts of the UK and probably some new administration for goods going the other way.
The UK and EU have yet to agree the nature and extent of the checks on goods after the transition period, with the committee expected to do much of the work on this issue.
The EU remains concerned that the UK is not moving quickly enough to prepare for new checks.
It said the UK needs to provide a detailed timetable for action.
"The exchanges in the specialised committee now urgently need to be followed up by tangible measures", it said.
The UK is understood to have offered the EU evidence that it has begun implementing the deal.
However, that evidence relates to the citizens' rights elements of the deal rather than the trade and customs parts.
The discussions are also understood to have touched on the checks which already happen on trade from GB to Northern Ireland.
Those are mainly checks on livestock when they arrive at Larne harbour.
The two sides remain at odds on the issue of whether the EU should have have permanent premises in Belfast, from where they could oversee the implementation of the deal.
The EU says the deal allows for such a premises but the UK disagrees.