The European Union’s chief negotiator Michel Barnier has expressed concern about progress so far, as the third round of Brexit talks gets under way.
He warned that UK “ambiguity” must be removed and progress on “separation” issues made before any talks on the future EU-UK relationship.
For his part, UK Brexit Secretary David Davis said both sides had to show “flexibility and imagination”.
The two men spoke at a brief media call before the talks got started.
Both sides have said there is no real prospect of a breakthrough in this third round of talks.
BBC Europe correspondent Kevin Connolly said the mood for the talks had been set by a series of briefings “that betray a good deal of mutual exasperation”.
The EU has accused the UK side of “magical thinking” on the issue of the Irish border.
And the UK has said the Europeans are “massively over-egging” their financial demands on the UK in terms of an “exit” bill.
The third part of the main “separation” issues to be resolved is the rights of EU citizens in the UK and UK citizens in the EU after Brexit.
Mr Davis said that the UK’s firm goal remained to secure a “mutually beneficial” agreement that works for people and businesses across Europe.
He said this week’s “technical talks” would build on the July round and the papers the UK has recently published on its vision for its withdrawal from the EU.
“For the UK, the week ahead is about driving forward the technical discussions across all the issues.
“We want to lock in the points where we agree, unpick the areas where we disagree, and make further progress on a range of issues.”
Mr Barnier said on Twitter last week that the focus of the third round of negotiations would be “orderly withdrawal”.
The EU position had been “clear and transparent” since day one, he said. It was “essential” to make progress on citizens’ rights, settling accounts and the Irish border, he added.
Officials are now set to meet in working groups to discuss the detail behind each side’s proposals, before the round is closed by Mr Davis and Mr Barnier on Thursday.
Setting the scene – political correspondent Chris Mason
Fairly technical. No big bang. No breakthrough. But no breakdown – 10 words that don’t make for a great headline, I’m afraid. But it is the mood music around the Brexit talks resuming in Brussels.
Meanwhile, at Westminster, there is “a new politics around Brexit” according to Phillip Blond, the director of the centre right Respublica think tank on BBC Breakfast this morning.
He was talking about Labour’s policy shift: promising to stay in the single market and a customs union for a transition period after Brexit.
The reaction, on Radio 5 live’s Your Call this morning, suggests the instinct of those on the winning side of a referendum is that such a vote should mark the end of the conversation on a subject: the people have spoken and we should leave, and leave properly.
And those on the losing side think it should mark the start of a conversation: about what leave should look like, or whether we should leave at all.
The UK is set to leave the EU by the end of March 2019, following last year’s referendum vote.
In the first phase of negotiations, British and EU officials are meeting each month for four days in Brussels.
On Sunday, Labour said it would keep the UK in the single market and customs union for a transitional period after leaving the EU.
It said this was needed to avoid a “cliff edge” for the economy.
Meanwhile, the British Chambers of Commerce and the Association of German Chambers of Commerce have jointly pleaded with politicians to put economic interests first in their Brexit discussions.