Post-Brexit talks have entered a "critical phase" and the coming days will show whether a deal can be reached on fair trade rules and fishing, EU chiefs said Thursday.

EU Commission chief Ursula von der Leyen and EU Council boss Charles Michel said that progress had been made since negotiations resumed in a more intense format.

But EU and UK negotiators Michel Barnier and David Frost have yet to agree on how to ensure a "level-playing field" for businesses and how to assign fishing rights in UK waters.

"We are in a critical phase now," Von der Leyen told reporters, at a video news conference after talks with Canada's Prime Minister Justin Trudeau.  

"And it's boiling down indeed to the two topics that are the most important that is the level playing field on one side and the fisheries on the other side."

Von der Leyen said Barnier still has Brussels' full support, but that the EU is insisting on a "dispute mechanism" to solve problems after Britain leaves the EU single market. 

And she said that this was the "sine qua non to have for the UK a access to the single market that is quota free and tariff free".

"This is crucial," she said. "This will be, in the next days, the field to work deep into."

Michel, whose council represents EU national leaders, said: "We will see, in the next days, what are we hope the positive developments."

Canada already has a trade agreement with the EU, and Trudeau was asked whether Ottawa would be able to agree a similar deal with the UK before the end of the year.

"I think you've heard that there are significant complexities that UK is grappling with in terms of negotiating the post Brexit-agreement with the European Union," he said. 

"I can highlight that it's fairly straightforward with Canada, and we are certainly very optimistic in the ability to see things roll over smoothly."