BRUSSELS - The European Union on Monday warned Britain time was running out to seal a Brexit deal this autumn and ensure London does not crash out of the bloc next March, adding to pressure on the embattled Prime Minister Theresa May.

Diplomats and officials in Brussels note little progress in Brexit negotiations since the EU leaders last met, raising doubt about whether the bloc and London would be able to mark another milestone at the next top-level summit due on June 28-29.

"We are concerned that there is no clear stance, no clear position from the British. The clock is ticking," German EU Minister Michael Roth told reporters on arriving for talks with EU peers in Brussels.

"We need now to be making substantial progress, but that is not happening. What is worrying us in particular is the Northern Ireland question where we expect a substantial accommodation from the British side."

At home, May is stuck between a rock and a hard place with staunch Brexit supporters pushing to severe ties with the EU and others advocating keeping close customs cooperation with the bloc to smooth trade.

Both sides of the Irish border worry that reinstating a physical border between EU-member Ireland and Britain's province of Northern Ireland could revive violence.

Other outstanding issues include ensuring expatriate rights, agreeing on security cooperation and trade rules after Brexit, to ensure a deal is in place when Britain leaves in March 2019, and an adaptation period ends at the end of 2020.

"From the European side, we can say that we have made clear our position and we hope that the British government will enter negotiations also in a full way," said Austria's Gernot Blumel, echoing a long-standing EU complaint that Britain has not made its position clear in detail on parts of the negotiations.

With May's cabinet, her ruling Conservative party and the British split on the matter, that has proven difficult to achieve.

The Brexit schedule is now coming under pressure again, sources said, partly an EU negotiating strategy before the June summit but mostly due to lack of substantial headway in the talks.

Dutch Foreign Minister Stef Blok said it was too early to discuss an extension of the timeline, but added: "The aim is now to conclude a deal in the time schedule that has been agreed on ... I very much hope we will agree but there are no guarantees, unfortunately."

Reuters