Dominic Raab has set out an uncompromising UK stance before key Brexit talks, insisting the EU has been given sufficient details of alternatives to the Irish backstop and that it must now show “the political will” to make a deal.

With Boris Johnson due to meet Jean-Claude Juncker for a working lunch in Luxembourg, the foreign secretary argued that the EU complaint that it remained in the dark about UK Brexit policy specifics was “political posturing”.

However, this argument seems likely to baffle and frustrate officials in Brussels.

It is 26 days since Johnson met Angela Merkel in Berlin, where the German chancellor said the prime minister had 30 days to persuade the EU there was a viable alternative to the backstop, the insurance policy to prevent a hard border on the island of Ireland. Yet EU sources insist Johnson’s envoy, David Frost, has barely started to touch on concrete proposals.

Writing for his former employer, the Daily Telegraph, on Monday, Johnson expressed optimism for a deal, but offered no details, saying: “I believe passionately that we can do it, and I believe that such an agreement is in the interests not just of the UK but of our European friends. We are working flat out to get one.”

Describing Johnson’s meeting on Monday with the European commission president as “an important milestone” in the talks, Raab told BBC Radio 4’s Today programme that complaints about a lack of detail were “just part and parcel of the tactical posturing that goes on in any negotiation”.

“The EU know our position. Lots of the detail has been talked through at both technical and political level, and actually the framework, as I have just described it, is very clear.

“Of course, the nature of these negotiations is that there will be a tendency to try and rubbish things we put forward in order to exact further demands. We’re not going to get involved in that.”

Raab argued that the EU’s complaints about a lack of detail masked an unwillingness in Brussels to move on its own positions. “We’ve been clear on all of this in the talks, and we’ll continue to flesh out and be able to respond to any questions.

“But of course at some point the EU is going to have to put up it hands and say: ‘OK, we know that there’s the requirement for some pragmatism and flexibility to get this deal over the line.’”

Johnson has until the European council summit in mid-October to reach a deal or he will be compelled under a newly passed backbench-instigated law to seek an extension to the Brexit deadline beyond 31 October, something he has vowed he will not do.

Downing Street’s hopes of a deal centre on finding a technical fix under which Northern Ireland would remain converged with the Republic of Ireland in some sectors, notably food and agriculture, while arrangements for other goods would involve avoiding checkpoints at the border.

This operation would only come into force at the end of a transition period triggered by a deal, currently slated to finish in December 2020. On Sunday the Brexit secretary, Stephen Barclay, argued this deadline could be extended for a year or two to give more flexibility.

But this idea was dismissed by Raab: “No. It’s not something under consideration.”

He argued repeatedly that it was up to the EU to shift.

“We want to see some finality to this,” he said. “It has got to that moment in time, as we lead into the October council, for the decisions to be made. We’re willing and able to do a deal on the grounds I’ve described. The question now is whether the EU has got the political will to meet us to get that deal, which is good for both sides.”

On Sunday, an EU source expressed exasperation at what they said was a continued lack of detailed negotiation, saying Johnson seemed more focused on political stunts, such as using a newspaper interview to compare himself to the Incredible Hulk.

They said: “It all makes it look like it’s a bit of a joke. We are talking about something extremely serious. The consequences of no deal will be extremely serious and it looks like this is being treated as a game in which you are the hero sort of story rather than [dealing] with real lives.”