Britain and the EU have taken "decisive steps forward' in Brexit talks after Prime Minister Theresa May's speech last week but more work needs to be done, negotiators said Thursday.

Speaking after the fourth round of talks in Brussels, EU negotiator Michel Barnier and his British counterpart David Davis said they had achieved more clarity on the details of May's speech in Florence.

But Barnier warned that they were "not there yet", with time running out to achieve sufficient progress to get EU leaders to agree to unlock discussions on a future relationship after Britain leaves in March 2019.

"Thanks to the constructive and determined manner in which both sides have conducted these negotiations I believe we are making decisive steps forward,"

Davis told a news conference alongside Barnier at the European Commission's headquarters.

"After four rounds when I look across the full range of issues to do with our withdrawal from the EU I'm clear we have made considerable progress on the issues that matter," added Davis.

The EU insists on making sufficient progress on three key divorce issues:

Britain's exit bill, the fate of Northern Ireland, and the rights of three million EU citizens living in Britain after its departure from the bloc.

May's Florence speech offered key concessions as well as proposing a transition period of around two years after Brexit to allow businesses to adjust to the new situation.

"The prime minister's speech in Florence has created a new dynamic in our negotiations and we have felt this during the negotiations this week," Barnier said.

"We managed to create clarity on some points, on others however more work remains to be done, and we are not there yet." (AFP)