David Davis was speaking in the US after a tense round of Brexit talks in Brussels
The UK's Brexit minister has conceded Britain had been put under pressure to resolve the issue of the Brexit 'divorce bill', which EU negotiators insist is agreed upon before talks can move on to the next phase.
David Davis addressed the US Chamber of Commerce in Washington, shortly after returning from what he admitted had been a "tense" round of talks in Brussels.
However, he insisted he remains a "determined optimist" over the possibility of a positive deal.
He would not be drawn on an interview by his colleague Liam Fox, who on Friday morning accused EU officials of "blackmailing" Britain over the divorce bill, which will reflect the UK's existing financial commitments to the EU.
Mr Davis said he would not comment on the views of other ministers, but said: "We are in a difficult and tough, complicated negotiation.
"I have said from the beginning, it will be turbulent. What we're having at the moment is the first ripple and there will be many more ripples along the way."
In his speech, Mr Davis also tried to reassure a domestic and European audience that Britain would not engage in a "regulatory race-to-the-bottom" by slashing quality standards or wages, in order to compete with emerging economies such as China.
The Brexit Secretary painted an optimistic picture of Britain's future outside the EU, and reiterated his government's desire to agree trade and customs arrangements with the bloc which are as "seamless and frictionless" as possible.
He said: "There are many that doubt this is possible but when I spoke to my European counterpart Michel Barnier I said to him once: 'The pessimist sees difficulty in every opportunity, the optimist sees opportunity in every difficulty'.
"And as ever on that front I am a determined optimist in this - we will get to those opportunities. Because fundamentally I believe that a good deal is in the interests of both the United Kingdom and the European Union and of the entire global community."
Mr Davis returned from the third round of exit talks in Brussels on Thursday, insisting at a terse news conference that "some concrete progress" had been made - only for Mr Barnier to retort that he thought there had been "no decisive progress" on key issues.