The British leader told reporters on Tuesday he remains "optimistic" a deal can be secured but warned that "it's looking very, very difficult at the moment."
British Prime Minister Boris Johnson warned on Tuesday that securing a Brexit deal is "looking very, very difficult" as he prepares to go to Brussels later this week to meet with Commission chief Ursula von der Leyen.
Johnson told reporters that "you've got to be optimistic, you've got to believe that the power of sweet reason to get this thing over the line".
He tempered that, however, by adding: "But I've got to tell you, it's looking very, very difficult at the moment."
"I've got to be honest with you, I think that the situation at the moment is very tricky. I think that our friends have just got to understand that the UK has left the European Union in order to be able to exercise democratic control over the way we do things," he said.
"Then there's also the issue of fisheries where we're a long way apart still. But you know, hope springs eternal and I'll do my best to sort it out if we can," he added
Johnson and von der Leyen announced on Monday evening, following their second phone conversation in 48 hours, that they were to meet face to face in Brussels in the coming days in a last-ditch attempt to bridge the remaining significant gaps between the two sides to strike a post-Brexit deal.
"As agreed on Saturday, we took stock of the ongoing negotiations. We agreed that the conditions for finalising an agreement are not there due to the remaining significant differences on three critical issues: level playing field, governance and fisheries," they wrote.
Michel Barnier, the EU's chief negotiator, and his British counterpart David Frost have been tasked with preparing "an overview of the remaining differences to be discussed."
Barnier, who briefed EU ministers on the state of the negotiations on Tuesday morning, said there is "full unity" among member states.
"We will never sacrifice our future for the present. Access to our market comes with conditions," he went on. He added that he is "working closely" with Frost to prepare for the Johnson-Von der Leyen meeting.
Barnier and Frost are currently locked in talks for the second week in a row, this time in Brussels. Last week's round of negotiations in the British capital ended with both men once more deploring "significant divergences".
The UK officially exited the EU on January 31 and the transition period is set to expire at the end of the year meaning the country will no longer have to abide by EU laws and will no longer enjoy the same access to the single market and customs union.
Both sides had hoped to reach a deal by mid-October in order to leave enough time for respective parliaments to ratify it but the deadline has been continuously pushed back.
Hopes now are for an agreement to be struck before Thursday so it can be presented to European leaders who will gather then for a Council meeting.
According to the Irish Foreign Minister, Simon Coveney, the "intensive negotiations" of the past two days have yielded "no progress".
"On fish, if anything, new problems are being landed on the table rather than solutions," he told broadcaster RTE on Monday evening.
"There is a great deal of frustration on the EU side not just within the EU negotiating team, Michel Barnier's team, but also across member states.
"We are really at a very delicate point in these negotiations now and without political intervention from the very top, ie from the Prime Minister and the President, then I think people are increasingly pessimistic that the negotiating teams can get this concluded successfully," he added.
He described as "a positive" the UK's government concession earlier in the day that it would be willing to drop contentious clauses in its Internal Market Bill that would allow it to override parts of the Withdrawal Agreement and thus breach international law.
"In some ways, they're simply taking something off the table that in the EU's view should never have been on the table but having said that I think it's important to welcome positives if they're there," he said.
For experts, Johnson's upcoming trip to the EU's capital seems to indicate he is willing to offer more concessions to secure a deal.
David Hening, the UK director of the European Centre for International Political Economy think tank, said following the announcement that "the needle moves slightly towards deal, but only slightly and because the PM is showing he wants a deal."
"The PM will, I suspect, believe he can get the major concessions he wants if going to Brussels in person. He still believes the EU will blink. And there will be goodies on offer from the EU. But not in the areas of core disagreement. That is how the EU seals trade deals," he wrote on Twitter.
For the European Policy Centre's chief executive and chief economist, Fabian Zuleeg, the latest announcement "must imply it signals that Johnson is willing to concede."
He added, however: "Find it hard to believe, he would probably be no longer PM by the time he gats back off the Eurostar."