Theresa May’s efforts to find a compromise on the Irish border that will unblock Brexit talks struggled on, with both sides digging in on Thursday as the clock ticks down to a deadline next week.

Irish Foreign Minister Simon Coveney rejected “creative ambiguity” on the intractable issue and called for Britain to come up with credible language to guarantee that Brexit won’t create a hard border on the island of Ireland. Northern Ireland’s Democratic Unionists, who have more clout than ever as they prop up May’s government in London, reiterated their red lines in a firmly worded statement.

May wants to find a solution by Monday, when she’s due to have lunch with European Commission President Jean-Claude Juncker. That meeting is meant to pave the way for an agreement from European leaders on Dec. 14 that Brexit talks can move on from the divorce to the crucial trading arrangements for after the split. Businesses are clamoring for a December deal and the pound has rallied on hopes there will be one.

“Hopefully we will make progress that will allow us to move onto phase two in the middle of December,” Coveney told lawmakers in Dublin on Thursday. “If it is not possible to do that, so be it.”

Double Bind

May needs to find a way of wording a commitment to the EU that Brexit won’t mean a hard barrier goes up between Northern Ireland and the Republic of Ireland when the 300-mile line dividing them becomes the U.K.’s new frontier with the EU. The issue is the main obstacle in talks after an outline deal on the financial settlement was reached.

Reiterating commitments to the Good Friday Agreement that brought peace to Ireland after decades of violence could be part of the solution, according to four European officials. But Ireland is insisting on a written commitment that goes further, and makes sure regulations on each side of the border won’t diverge significantly after Brexit, one of the officials said.

Ireland wants no border at all — for historic, political and economic reasons — and the EU has adopted the same stance. But no border means keeping Northern Ireland aligned with the regulations that apply in the Republic, and that could mean erecting barriers between Northern Ireland and mainland Britain — a red line for the DUP.

“Her Majesty’s Government have a clear understanding that the DUP will not countenance any arrangement that could lead to a new border being created in the Irish Sea,” DUP leader Arlene Foster, said in a statement.

DUP lawmaker Sammy Wilson told the BBC that attempts to “placate Dublin and the EU” could jeopardise the party’s support for May’s government. Speaking in private, one European official said the chances of an agreement on the Irish issue by next week were 60-40. Another European official saw a 50-50 chance.

The Times of London reported that the solution could hinge on giving Northern Ireland more powers locally over customs, energy and agriculture as a way to keep rules the same on each side of the border after Brexit. The U.K. proposal commits it to working to avoid regulatory divergence on the island of Ireland, the paper said.

That probably wouldn’t go far enough, according to a European official, familiar with Irish thinking. DUP lawmaker Ian Paisley, said on Thursday that Brexit will mean Northern Ireland gets different rules to the Republic on matters such as agriculture.

“There’s areas where frankly divergence will come about because we in Northern Ireland, being part of the United Kingdom, believe we’re going in a different direction, for example on agriculture,” DUP lawmaker Ian Paisley said in an RTE interview on Thursday. “Why would we hold to certain policies that would hold us back.”

That could be a major obstacle. For now, farm animals roam freely across the border, which is all but invisible thanks to the EU, its single market, and its common agricultural policy. Brexit will mean a border probably has to go up somewhere as the U.K. plans to leave Europe’s single market and customs union. The U.K. also wants to strike trade deals with other countries: if U.S. food imports start coming freely into Northern Ireland then a border will be need to block them crossing the Irish border into the EU.