Andy Burnham has to be serious about devolution and understand that Wales is a nation and “not a region in England”, according to the country’s first minister.

Rhun ap Iorwerth, who is the first non-Labour leader of the Senedd since devolution, told the Guardian that while he was encouraged by initial discussions with the new UK prime minister, he was wary about Burnham being captured by a consensus Westminster view against relinquishing further powers.

Separately in the interview, ap Iorwerth said his party, Plaid Cymru, would be willing to cooperate with Labour in the future to keep Reform UK out of power, as he would always want to block the “destructive politics” of Nigel Farage’s party.

Ap Iorwerth became Wales’s first minister in May, days after he led Plaid Cymru to victory in Senedd elections, breaking a 100-year period of Labour dominance, which included Labour being the biggest party in every vote to the Welsh parliament since it was created in 1999.

He has pushed for more powers to be given to his government, particularly over policing and the justice system, where Wales is the only UK nation not to have control.

Burnham’s background as Greater Manchester mayor was a positive, ap Iorwerth said: “He clearly understands the benefits of getting devolution right for Manchester. I’ve had really positive, constructive discussions with him.

“He now needs to know what devolution means for Wales, and it’s very, very different. We’re a nation. We’re not a region of England. There are different needs. We will work closely with the UK government on what empowering communities means. But it’s a totally different model, and it’s really important that Andy Burnham doesn’t confuse the two.”

Speaking in London after meetings with UK government officials, ap Iorwerth said, although Burnham had assured him he would “not bypass devolution”, he was concerned about institutional resistance to handing over police and justice powers.

In July, Burnham’s Welsh secretary, Stephen Kinnock, said he was not “entirely convinced” about the idea of devolving powers such as policing. Ap Iorwerth said: “The arguments for the devolution of policing and justice are very, very strong. Commission after commission and study after study have concluded this really needs to happen.

“Andy Burnham had more control over policing as the mayor of Manchester than I do as first minister of Wales, and it’s just not right. Burnham gives me the impression, and I believe him, that he’s ready to take a different approach.

“What I’m also acutely aware of is that those forces around him are probably equally as averse to devolving policing and justice as they have been under previous Labour leaders.”

With 43 Senedd seats, Plaid is the biggest party in the expanded 96-member chamber, with Reform forming the opposition. Reform enjoyed “a very significant result” in May in going from two members – both defectors from the Conservatives – to 34, ap Iorwerth said.

However, he pledged to stand against what he termed Reform’s “kind of politics that runs contrary to my core beliefs of cooperation and tolerance and supporting each other and looking after the weak”.

Asked if Plaid would potentially cooperate with Labour to block a potential Reform government, whether in Cardiff or Westminster, he said it would: “It’s not about keeping a political party out, it’s about keeping a kind of politics out, the negative influence that it has, and of course we do and will cooperate with Labour.”

Related: Plaid Cymru has forged a brand of inclusive nationalism. That’s why it beat Reform in Wales | Rhiannon Lucy Cosslett

At a press conference earlier this month to mark the start of the new Senedd session, ap Iorwerth took a somewhat downbeat approach, warning about a “very challenging” financial picture, with a real-terms decline in both day-to-day and capital spending this year.

Asked if he risked mimicking Keir Starmer’s sometimes gloomy approach in assessing the task of government, ap Iorwerth said that was not his intention: “I would certainly hope not, because I think a mistake Keir Starmer made was that the government almost gave up on its values, gave up on its beliefs under that administration, because things were difficult.

“We’ve taken a completely different approach, and if there was a sense of gloom, it was certainly not meant to be like that.”

On Monday, ap Iorwerth is due to hold a meeting in Cardiff with his equivalents in Scotland and Northern Ireland, John Swinney of the Scottish National party and Michelle O’Neill of Sinn Féin, an event labelled by one Telegraph columnist as a “sinister cabal … plotting nothing less than the end of the United Kingdom”.

Asked about this description, ap Iorwerth smiles. “It’s just not the way that I see it,” he says. “I don’t come from the base of working, either ourselves or with any other partners, to break up the UK. I want to build up my nation. That’s all it’s about.”

Wales is, he says, on a “constitutional journey”, one in which he sees the ultimate destination as being independence – but without any set timetable or rush: “That’s in the hands of the people of Wales.”