LONDON, Dec. 8 (Xinhua) -- British Prime Minister Boris Johnson said Tuesday that the post-Brexit trade talks between Britain and the European Union (EU) are "very difficult" at the moment.

Johnson made the remarks ahead of his visit to Brussels this week for a crunch meeting with EU leaders in a bid to seek a breakthrough in the lengthy negotiations.

"You've got to be optimistic, you've got to believe there's the power of sweet reason to get this thing over the line," the prime minister told Sky News.

"But I've got to tell you it's looking very, very difficult at the moment," he said.

Speaking to the BBC on Tuesday, Johnson said: "We will see where we get to in the course of the next two days, but I think the UK government's position is that we are willing to engage at any level, political or otherwise, we are willing to try anything."

"But there are just limits beyond which no sensible, independent government or country could go and people have got to understand that," he added.

The prime minister is expected to travel to Brussels on Thursday, the BBC said Tuesday.

In a bid to unlock the stalemate in post-Brexit trade talks, Johnson and European Commission President Ursula von der Leyen made a telephone call on Monday, the second one in 48 hours.

During the phone conversation, they said that the conditions for finalizing a trade agreement "are not there due to the remaining significant differences on three critical issues: level playing field, governance and fisheries" between the two sides.

"As agreed on Saturday, we took stock today of the ongoing negotiations," the two leaders said in a statement following the phone call.

"We asked our chief negotiators and their teams to prepare an overview of the remaining differences to be discussed in a physical meeting in Brussels in the coming days," the statement added.

The two leaders had phone conversations several times instead of face-to-face discussions due to the COVID-19 pandemic.

To bring life back to normal, countries including Germany, China, Russia, and the United States are racing against time to find a vaccine.

The trade negotiations are at a crucial stage as time is running out for both sides to secure a deal before the Brexit transition period expires at the end of the year.

Failure to reach a free trade agreement means bilateral trade will fall back on World Trade Organization (WTO) rules in 2021.