Sir Keir Starmer said he could not give Port Talbot steelworkers “false hope” ahead of the closure of the town’s final blast furnace next month.
The British Labor government is in talks to save jobs at Tata Steel, where thousands face redundancies.
The prime minister said ministers would do “everything we can” but added: “I’m not going to pretend it’s not going to be difficult.”
Welsh Conservative Senedd leader Andrew RT Davies claimed the Labor minister had led Port Talbot “up the garden path”.
A Plaid Cymru MP called Sir Keir’s comments “unfortunate”.
The British Labor leader spoke to reporters at a windfarm in Carmarthenshire, as part of a two-day trip to Wales.
Sir Keir met Eluned Morgan at Cathays Park, in Cardiff, for his first meeting with her since she became first minister there.
The Labor leader claims he has “turbocharged” the party’s performance on steel since he came to power.
Asked if he risked giving Port Talbot steelworkers false hope, he said: “No.
“We are working hard on this, the first minister and I together, because it is very important to do everything to keep the job.”
He then added: “I don’t want to give anyone false hope.
“I’m not going to give up on making sure that those in the supply chain have the money they need to diversify, where they need to diversify, so we put the money in just seven days ago.
“It is very important that we have steel production in south Wales, it is very important that the government is committed.
“These are difficult times, there’s no doubt, but we’ll do everything we can to keep those jobs.”
Analysis
According to Huw Thomas, BBC Wales business reporter
The Labor opposition has been accused by the Conservative government of giving false hope to Tata Steel workers, and it is an accusation Sir Keir Starmer continues to deny as Prime Minister.
While it is true that the new Labor government has announced funding for supply chain businesses, this is being shaped in a cross-party transition space where remits and budgets were managed by the previous Westminster administration.
Tata Steel, meanwhile, is moving ahead with its plans.
It has invited workers to volunteer for redundancy, and will close the second of two blast furnaces in September.
Since the steel mill announced its original proposal earlier this year, little has changed in its overall plan to cut 2,800 jobs and close its Port Talbot operation in the autumn.
That is despite industrial action and political pressure.
With just a few weeks left, the blueprint for the Steel’s lineup appears to be unchanged. Any additional investment or job creation may be secondary, and small in size, compared to what is lost.
Eluned Morgan told BBC Wales: “We all know very well that there is a sword of Damocles in Tata’s situation.”
“The situation is difficult, and you know, we have to be ready for all options.”
The first tranche of a £100m fund to help workers and companies affected by the restructuring plan was announced last week.
The £13.5m announced is to support local companies whose main customer is Tata Steel find new markets and help workers find new jobs, access training and gain skills and qualifications in areas where there are vacancies.
No.10 has promised to reset the relationship between the Welsh and English governments, with the same party now running both.
Asked if there would be extra cash for the Welsh government to put on the waiting list, Ms Morgan said: “There are a number of issues that we discussed yesterday.
“Because they’re going to put money on the table in England to lower that waiting list, we’re going to get money as a result, we’re going to decide to use it as we see fit.
“I’m very keen to make sure we use that money to bring the waiting list down.”
He said the UK government could learn from the Welsh government. “We have changed the rules, for example, around GPs and how they should see patients. We have changed their contracts. They have not done so in England,” he said.
Ms Morgan added: “We will work together on the things we agree on, but we have to understand that there are some things that are being pushed down.”
Andrew RT Davies said: “It’s clear now that the Conservatives in government have any real hope of keeping steel production in Port Talbot with arc furnaces.”
He added: “Steel workers and the community in Port Talbot have been misled by Labor and led down the garden path”.
Ann Davies, Plaid Cymru MP for Carmarthen, said Sir Keir’s comments were “unfortunate”. He said that before the election the Labor leader had said “there is a plan B”.
“It is, really, we have to look at importing steel, say from China, from all over the world … instead of looking at a viable alternative to Port Talbot.”