European Union's Brexit negotiator Michel Barnier takes part in the EU Commission's weekly college meeting in Brussels, Belgium, October 10, 2018. REUTERS/Yves Herman

BRUSSELS (Reuters) - The thorny problem of how to ensure Brexit does not mean frontier checks between Ireland and Northern Ireland is among issues holding up a deal on Britain’s withdrawal, EU negotiator Michel Barnier said on Sunday.

Tweeting after he met British Brexit Secretary Dominic Raab following a weekend of talks between officials, Barnier said: “Despite intense efforts, some key issues are still open, including the backstop for IE/NI to avoid a hard border.”

The EU insists that Britain give assurances that, in the event future negotiations fail to ensure a UK-EU trade deal which can avoid a hard border, the British province of Northern Ireland should remain inside the EU’s economic rules. British Prime Minister Theresa May and her allies in Belfast reject that.