SALZBURG, Austria — EU leaders will on Thursday make preparations for the final stretch of Brexit negotiations, after clashing with Britain over how to bridge their differences to reach a divorce deal.

Leaders of the 27 other European Union countries will meet without British Prime Minister Theresa May in Salzburg, during the first of three summits in successive months that Brussels hopes will yield an agreement.

But as they arrived for a pre-summit dinner in the Austrian city on Wednesday evening, both sides called on the other to make further concessions.

EU Council President Donald Tusk said that despite progress in some areas, on the two thorniest issues of the Irish border and post-Brexit trade ties, "the UK’s proposals will need to be reworked".

May, who is under intense pressure from Brexiteers back home, retorted that she had already made compromises and it was now the turn of Brussels.

"If we are going to achieve a successful conclusion then, just as the UK has evolved its position, the EU will need to evolve its position too," she said.

Both sides had been aiming for an October EU summit as the deadline to reach an agreement, to allow time for the deal to be ratified by British and European parliaments before Brexit in March.

But with the talks deadlocked, Tusk is seeking approval on Thursday for another summit in November -- when he warned there must be a deal, to avert a "catastrophe" of Britain crashing out of the bloc.

’Get this deal done’

The summit dinner was dominated by discussions over how to deal with irregular migration into Europe, but May gave a short speech on Brexit at the end, officials said.

She pressed her case for a Brexit deal, telling fellow leaders: "The onus is now on all of us to get this deal done."

For the first time, she presented to the group her so-called Chequers plan for the post-Brexit trading relationship, which was published in July.

Her proposal to follow EU rules on trade in goods has provoked a fierce backlash among eurosceptics in her Conservative party, renewing speculation of a challenge to her leadership.

EU chief negotiator Michel Barnier has also been deeply critical of the plan, which May hopes could form the basis of a political agreement on trade to be included in the divorce deal.

British officials insist however that it is the only way to protect existing trade after Brexit while also resolving the Irish issue.

Irish border talks

Much of the divorce deal is agreed, but a key sticking point is how to avoid a "hard border" between British-ruled Northern Ireland and the Irish Republic when London leaves the EU single market and customs union.

There are fears that frontier checks would disrupt trade and could undermine the 1998 peace deal on the island.

Europe is insisting on a fall-back plan, a "backstop", that would keep Northern Ireland in the customs union under EU rules while a future trade relationship is negotiated.

London has rejected this, saying it would create a border in the Irish Sea that threatens its territorial integrity, but offered only a partial proposal in return.

May will meet with Irish Premier Leo Varadkar before the Brexit talks on Thursday, and Tusk will brief her afterwards on what the EU27 discussed.

Diplomats warned before Salzburg that there would be little headway as both sides act cautiously before May’s Conservatives begin their annual conference on September 30.

But there were signs of some movement on Ireland, after Barnier suggested any checks between Northern Ireland and the rest of the UK could be away from the border.

May welcomed his willingness to "find a new solution", and conceded that some checks were already carried out in the Irish Sea, on agricultural products.

But she repeated her warning that Britain would not agree to a "legal separation of the United Kingdom into two customs territories". — AFP