UPDATE 1-EU, UK step up N Ireland talks as EU continues legal action

(Combines Frost and Sefcovic statements after Thursday meeting)

By Philip Blenkinsop and Michael Holden

BRUSSELS/LONDON, April 16 (Reuters) - The European Union said on Friday that Britain should not change trading rules in Northern Ireland on its own and that the bloc would continue its legal case over unilateral British action in the province for as long as necessary.

European Commission vice president Maros Sefcovic, who hosted UK negotiator David Frost for talks on Thursday evening, said only agreements by joint bodies established by the Brexit divorce deal could provide stability in Northern Ireland.

The British-ruled province has remained in the EU single market for goods since Brexit to ensure an open border with EU member Ireland and so requires checks on goods coming from other parts of the United Kingdom.

Britain in March unilaterally extended a grace period on certain checks to minimise supply disruption, a move Brussels said breached the Brexit divorce deal known as the Withdrawal Agreement and the specific protocol on Ireland/Northern Ireland.

Sefcovic said in a statement on Friday there was no space for unilateral action. He said both sides had to agree on how to comply fully with the protocol, including "clear end-points, deadlines, milestones and the means to measure progress".

Frost said the British government was committed to working through joint bodies and that all solutions had to respect the Good Friday peace agreement "in all its dimensions" and to ensure minimal disruption to everyday lives in Northern Ireland.

Both agreed that Thursday's discussion took place in a constructive atmosphere, that talks needed to intensify and that they would jointly engage with business groups, civil society and others in Northern Ireland.

Frost said some "positive momentum" had been established.

"But a number of difficult issues remained and it was important to continue to discuss them," the British government said in a statement. (Reporting by Michael Holden and Elizabeth Piper in London, Philip Blenkinsop and Jan Strupczewski in Brussels Editing by Gareth Jones)