Brexit negotiations “have been difficult” and “no solution has been identified” to the Irish backstop, the European Commission has said.
It comes after the latest talks between UK ministers and EU Brexit negotiator Michel Barnier in Brussels.
Commission spokesman Margaritis Schinas said the talks had taken place in a “constructive atmosphere” but there had been no breakthrough.
The UK is pushing for legally-binding changes to the EU deal.
Mr Schinas was speaking after Mr Barnier briefed the European Commission’s weekly meeting on the state of Brexit talks.
Speaking after talks with Mr Barnier, the UK’s Attorney General Geoffrey Cox said: “Both sides have exchanged robust, strong views. We’re now facing the real discussions. Talks will be resuming soon.”
He added: “We’re into the meat of the matter, we’ve put forward very reasonable proposals.”
The backstop is an insurance policy – designed to avoid a hard border “under all circumstances” – between Northern Ireland and the Irish Republic.
Prime Minister Theresa May is pinning her hopes on getting changes to it that will prevent the UK from being tied to EU customs rules if no permanent trade deal is agreed after Brexit.
She believes this would be enough to get MPs – who last month rejected her deal by an historic margin – to back her deal in a vote she has promised on or before 12 March.
But the EU has consistently refused to rewrite the deal it has struck with Mrs May, which is meant to ensure an orderly departure from the bloc on 29 March and pave the way for trade talks.
And Michel Barnier repeated that message to EU leaders, according to Mr Schinas.
“Discussions have been difficult and no solution has been identified to that is consistent with the withdrawal agreement, including the Northern Ireland protocol which, as you know, will not be reopened,” he said at a press conference in Brussels.
BBC Brussels reporter Adam Fleming explained that EU sources said the UK side couldn’t guarantee that whatever might end up being agreed in Brussels would even get through Parliament.
Mrs May is also hoping to attract votes from Labour MPs in Leave-voting areas of the UK, as she battles to get her deal through the Commons.
She is promising MPs a vote on any changes to workers’ rights after Brexit.
No 10 said Parliament would be given a say over whether to adopt any new protections introduced on the continent and to stay aligned with EU standards.
Labour MPs have been seeking assurances the UK will not fall behind EU standards after Brexit.
But trade unions said the MPs should not be “taken in by blatant window dressing” and the assurances on workers’ rights were “not worth the paper they are written on”.