UK Prime Minister Theresa May has urged European Union leaders to set out a plan for negotiations about Britain's exit from the bloc to move forward, but her counterparts called for more tangible details.

May's appeal on Thursday came at a Brussels summit where the 27 other leaders were set to decide that there had been insufficient progress on Brexit talks to open negotiations on a future trade deal until December.

Preliminary negotiations have focused on citizens' rights, a financial settlement and the Irish border. EU negotiators say they need more progress on those issues before talks on a future relationship can begin.

Still, May called for swift progress on the current stage of negotiations, with a view to achieving enough progress so her EU counterparts could approve the start of actual negotiations during a summit in December.

"I particularly ... want to see an urgency in reaching an agreement on citizens' rights," May said on the sidelines of the summit.

"I set out a few weeks ago in Florence a very bold and ambitious agenda and vision for our future partnership between the EU and the UK ... I look forward to us being able to progress that in the weeks ahead," added May, referring to a speech she gave in the Italian city last month, where she proposed a two-year transition after Brexit.

'The British must be clear'

However, several other EU leaders noted that there had not been sufficient progress and called for more details from the British side.

"What they [Britain] always say is that they want to continue to have the closest possible relationship with Europe and with Ireland," said Irish Prime Minister Leo Varadkar.

"We already have the closest possible relationship: it's called the European Union. And I think we need a little bit more detail - a lot more detail, in fact - on how you can square the idea of the closest possible relationship with the circle of the fact that they are departing from that."

Dutch Prime Minister Mark Rutte said May "hasn't been able to produce" any details on a possible financial settlement.

"I called her [May] and said: Listen, we need more clarity, specifically about the bill," Rutte told reporters.

"The British must be very clear on the exit bill."

French President Emmanuel Macron said that all 27 member states were strongly "united" in the conclusion that Brexit negotiations to date have not gotten far enough.

But German Chancellor Angela Merkel said the progress - while not yet sufficient - was encouraging.

Al Jazeera's Jonah Hull, reporting from the Brussels summit, said, "the EU is not convinced about sufficient progress" in the preliminary talks.

"They are not sold on Britain’s offer of meeting its outstanding liabilities and commitments to the tune of $20bn-plus over a two-year transition period - the figure they have in mind is likely tens of billions of dollars higher than that," he said.

After hearing a presentation by May on the state of the Brexit negotiations Thursday evening, the 27 other EU leaders are expected to approve a statement on Friday stating that they could not yet give a mandate to begin talks about a future relationship.

However, noting positive momentum in the talks, they will call for the start of internal preparatory talks, to speed up moving on to the next stage, which they are aiming to approve in December - pending sufficient progress.

"Putting people first". My open letter to EU citizens living in the UK #Brexit https://t.co/Lz3ayAKtOw — Theresa May (@theresa_may) October 19, 2017

Ahead of her trip to Brussels, May published an open letter to the three million European citizens living in Britain promising they would not be asked to leave after Brexit in March 2019.

She also engaged in a whirlwind of diplomacy before the summit, calling the French and German leaders, among others, and having dinner with European Commission chief Jean-Claude Juncker in Brussels on Monday.