The EU's chief Brexit negotiator says talks are looking at the "minutiae" of how a backstop for the Irish border could work.
Michel Barnier was speaking after a meeting with the Brexit Secretary Dominic Raab.
Both sides have agreed the need for a backstop but not how it should operate.
The EU has proposed a backstop that would mean Northern Ireland staying in the EU customs union, large parts of the single market and the EU VAT system.
The UK government has rejected it as a threat to the integrity of the UK and has suggested a backstop that would see the UK as a whole remaining aligned with the EU customs union for a limited time after 2020.
Mr Barnier said the EU has now asked the UK for additional data which will allow an assessment of "who, when, where and how" control and checks would be carried out.
He described it as "technical work" which needs to be carried out with precision.
He emphasised that without an agreed backstop the UK would be leaving the EU without a deal.
Mr Raab said that any solution must be workable for communities in Northern Ireland and the Republic of Ireland.
He described himself as "stubbornly optimistic" that a deal is within reach.