EU leaders agreed Friday to start internal work on the bloc's relationship with Britain after Brexit, giving some progress for embattled Prime Minister Theresa May to take back home.

EU President Donald Tusk said in a tweet the bloc's other 27 leaders meeting in Brussels had agreed to begin preparations for trade talks even though not enough progress has been made on the terms of the divorce.

"Brexit conclusions adopted. Leaders green-light internal EU27 preparations for 2nd phase," Tusk said on Twitter as EU leaders met without May to discuss the issue. It took the leaders 90 seconds to approve the conclusions, an EU source told AFP.

The toughest sticking point has been the bill Britain will pay as it leaves the EU club. European capitals are demanding detailed written commitments on finance before progressing to trade talks, fearing that Brexit will blow a hole in the bloc's budget.

In a move that risks being seen as a snub to the EU's gesture, May insisted once again on Friday that a detailed financial deal could only be reached once Britain's future relationship with the bloc was agreed.

"The full and final settlement will come as part of the final agreement that we're getting in relation to the future partnership. I think that's absolutely right," she said at the end of the Brussels summit.

German Chancellor Angela Merkel struck an optimistic note following a summit dinner on Thursday night where May addressed the leaders. Merkel said that despite delays in the negotiations, she could see "zero indications that we will not succeed" in reaching a final agreement.

- 'We have given a sign' -

Written conclusions approved by the leaders said the EU will delay the decision on opening the next phase of talks until the next summit in December, but they will agree to "start internal preparatory discussions" on trade and a possible transition deal.

A European diplomatic source said: "May asked for a sign, we have given a sign."

The slow progress of the negotiations, particularly on Britain's financial settlement, stoked fears that the country could leave the EU in March 2019 without a deal in place, risking economic and legal chaos.

Merkel, the bloc's most powerful leader, emphasised she did not want this, saying: "I want very clearly a deal and not some unpredictable solution, on this we are working very intensively."

The EU agrees that of the three key separation issues at stake, citizens' rights is the most advanced, but sticking points remain on the bill and Northern Ireland's border with Ireland.

- A deal to defend -

An EU source said that starting preparations on guidelines for the trade talks would save time if and when the political decision was taken to move forward in December.

A French presidency source added that "scoping work has already broadly started", referring to preparations on possible details of a deal.

May pressed her colleagues over a dinner on Thursday for "urgent" progress on a deal she could sell to voters and her own Conservative party, which is still divided over Brexit.

"There is increasingly a sense that we must work together to get to an outcome we can stand behind and defend to our people," she said, according to Downing Street.

A British official noted that May, who lost her parliamentary majority in the June election and is facing calls by some Conservatives to walk away from the talks, was "working against a difficult political backdrop".

Dutch Prime Minister Mark Rutte said May "pleaded her case well", but did not offer any more detail than in a speech she made in Florence in September.

He called for more "concrete" positions in the run-up to December.

European leaders have been increasingly vocal about their frustration at divisions in May's cabinet over Brexit, saying they are still unsure what Britain wants despite five rounds of negotiations.

On the poisonous issue of the Brexit bill, Germany and France insist there should be no impact on the EU's budget from Britain's departure.

In Florence, May promised to maintain Britain's contributions for two years after Brexit to complete the current EU budget period, totalling around 20 billion euros ($24 billion).

European Parliament chief Antonio Tajani dubbed that sum "peanuts" this week and said it should be nearer 50 billion or 60 billion euros.

- 'Find a solution' -

May engaged in a whirlwind of diplomacy with key European leaders ahead of the summit.

In a move to sway her colleagues on what is a key topic for them, she published an open letter to the three million European citizens living in Britain on Thursday, saying a deal to secure their post-Brexit rights was "within touching distance".

A German government source said progress on citizens' rights had gone "very, very far" and crucial future EU-British ties "outweigh the current dispute about finances."

The source concluded: "I believe everyone is aware of that and that is why we are optimistic that we are going to find a solution there -- with theatrical rumbling."