The EU has accused the UK of "wasting valuable time" in Brexit negotiations, adding how it "seems unlikely" a deal will be reached before the deadline.
Both sides agreed in their statements following the seventh round of talks that time is "short" and that little progress had been made.
Michel Barnier, the EU’s chief Brexit negotiator, told a press conference following the latest round of talks that "too often this week it felt as if we were going backwards more than forwards".
He added: “I simply do not understand why we are wasting valuable time.”
David Frost said the European Union is making it "unnecessarily difficult to make progress" in talks, with the bloc "insisting" that the UK accepts its position on state aid and fisheries before allowing talks to progress "in any other area".
He said an "agreement is still possible, it is still our goal, but it is clear that it will not be easy to achieve".
The EU wants a deal to be reached by October so it can be approved by the European Parliament before the transition period ends on December 31.
Mr Frost said: “Substantive work continues to be necessary across a range of different areas of potential UK-EU future co-operation if we are to deliver it.
“We have had useful discussions this week but there has been little progress.”
Mr Frost said the EU wants the UK to remain signed up to the Common Fisheries Policy, and agree continuity with EU state before a deal can be reached.
Both sides blamed the other for the lack of progress in talks.
The UK hit back at the accusation of time wasting, accusing the bloc’s position of having “frozen” progress.
A senior negotiating official for the UK said: “The process block now is the EU’s insistence that we must accept their position on state aid and fisheries before we can talk about anything else.
“Obviously we’re not going to do that. So it’s frozen.
“That doesn’t seem to us to be sensible. We should be talking about the many other areas, some of which are productive, some of which are more difficult, but we are ready to talk about anything and it’s not us that’s slowing it down.”
The EU's Mr Barnier said “no progress whatsoever” had been made on fisheries.
Speaking at a press conference following the latest round of talks, Mr Barnier said: “Ladies and gentlemen, the need for a level playing field is not going to go away, even if the UK continues to exist on a low-quality agreement on goods and services only.”
He added that “it is a non-negotiable pre-condition to grant access to our market of 450 million citizens”.
“We are asking for nothing more but nothing less, nothing less than what Prime Minister Boris Johnson committed to in our joint political declaration last October together with the 27 EU leaders," he said.
The idea of a level playing field is to ensure fairness between the two sides in areas such as labour, environment, climate, sustainable development and subsidy control.
“Apart from the question of a level playing field there are still many other areas where progress is needed and for example, obviously fisheries where we have made no progress whatsoever on the issues that matter.”
Mr Barnier added that other areas where more progress is needed include law enforcement “where we still struggle to agree on the necessary guarantees to protect citizens’ fundamental rights and personal data”.
He continued: “And also mobility and social security co-ordination where our positions are also driven far apart.”
The UK's Mr Frost said both sides will "continue to work hard to reach an agreement" and "remain in close contact" ahead of the next round of talks in London, beginning September 7.