The European Union and Britain say they held constructive post-Brexit talks but difficult issues remain over Northern Ireland trade.
European Commission Vice President Maros Sefcovic hosted British negotiator David Frost for talks in Brussels on Thursday after recent political tensions and rioting in Belfast.
In a statement, Mr Sefcovic said only jointly agreed solutions to trading issues would be acceptable and provide stability in the province.
"The vice president insisted on mutually agreed paths towards full compliance with the Protocol, which includes clear end-points, deadlines, milestones and the means to measure progress."
"The vice president reiterated the EU's commitment to the Protocol, which is the only way to protect the Good Friday (Belfast) Agreement and to preserve peace and stability, while avoiding a hard border on the island of Ireland and maintaining the integrity of the EU Single Market."
Legal action against Britain over unilateral action in Northern Ireland would continue "as long as necessary", the statement said.
The Commission earlier said the meeting was designed as a stock-taking exercise and to provide a steer for future talks, which will be held in a fortnight.
UK negotiator Mr Frost said there had been "positive momentum" in constructive discussions, but that difficult issues remained.
"Lord Frost said that the intensive discussions between the co-chairs of the Specialised Committee on the Protocol on Ireland / Northern Ireland in recent weeks had begun to clarify the outstanding issues, and some positive momentum had been established," the UK government said on Friday.
"But a number of difficult issues remained and it was important to continue to discuss them. He agreed there should be intensified contacts at all levels in the coming weeks.
“Lord Frost repeated the UK’s commitment to working through the joint bodies provided for by the withdrawal agreement."
On Thursday, the European Parliament's committee on relations with Britain voted overwhelmingly in favour of the post-Brexit trade and co-operation agreement, clearing the path to its final ratification.
They suspended voting in March in protest over British changes to trading arrangements in Northern Ireland, which Brussels says breach the terms of the Brexit withdrawal agreement.
The UK left the European Union on January 31 after years of tortuous negotiations over future relations but many details are unclear, leading to acrimony.
The EU's foreign affairs and trade committees backed the trade and co-operation agreement struck in December by 108 votes to one, with four abstentions, the parliament said.
The full chamber must still give its approval and, while it is clear the deal would receive majority backing, it is not certain that legislators will vote.
Parliament faces an end-of-April deadline but said it wants to see Britain move on implementing the Northern Ireland Protocol.
If there is no vote this month and provisional application of the agreement is not extended, then the trade deal would cease to apply, leaving Britain and the European Union to trade on World Trade Organisation terms.
