US Secretary of State John Kerry said Monday that a massive EU-US trade deal could soften any damage caused by Britain's exit from the European Union following last month's referendum.

The treaty, bogged down in negotiations, has "a very significant ability to act as a counter" to the negative effects of Britain's eventual divorce from the EU, Kerry told a press conference in Brussels.

"It's our job to make sure that we adequately inform people about the facts of how TTIP will actually work for the people of Europe," Kerry said.

"It will protect jobs, it will protect their regulatory rights, protect their abilities with respect to labour and the environment," he said.

Negotiators from the US and the EU are currently in trade talks to finalise the proposed Transatlantic Trade and Investment Partnership (TTIP) that would create the world's largest free trade area.

But the shock setback of Brexit and rising opposition to the deal in France and Germany have raised serious doubts that it will be achieved by the end of the year as hoped.

Kerry said that he would embark on a European tour in "the coming month ... to lay out the facts for people to understand exactly the positive side" of a potential deal.

Kerry's comments came just days after the top US trade official said that Britain's vote to break with the EU opens new questions over the deal.

Britain "is a very significant part of what makes TTIP attractive," said US Trade Representative Michael Froman on Thursday in Washington.

Those comments came amid strong opposition by activists who have opposed TTIP since negotiations began in 2013, believing it will only benefit multinationals and harm consumers.

Earlier this month, France's Prime Minister Manuel Valls said it would be "impossible" for the two sides to conclude negotiations on a trade deal by the end of 2016, which coincides with the end of the Obama administration.

Behind the scenes, top diplomats told AFP talks may be suspended until after the US presidential elections in November as well as elections in France and Germany next year.