Sir Keir Starmer has denied giving "false hope" to Port Talbot steel workers ahead of the planned closure of the town's last blast furnace next month.
The Labour UK government is in discussions to try to save jobs at Tata Steel, where thousands face redundancy.
The prime minister said the Labour UK government will do "everything we can" but added: "I'm not going to pretend it's anything other than tough."
He was speaking to journalists at a windfarm in Carmarthenshire, as part of a two day trip to Wales.
He met Eluned Morgan in Cathays Park, in Cardiff, for his first meeting with her since she became first minister on Monday.
Sir Keir claimed to have "turbocharged" the party's action on steel since he has been in power.
Asked if he was risking giving false hope to Port Talbot steel workers, he said: "No.
"We're working really hard on this, both the first minister and myself jointly on this, because it's so important we do everything we can to preserve those jobs."
He later added: "I don't want to give anybody false hope. I'm not going to give up on trying to preserve as many jobs as we can.
"I'm not going to give up on making sure that those in the supply chains have the money they need to diversify, where they need to diversify, which is why we put money behind this just seven days ago.
"It is very important that we have steel production here in south Wales, very important that the government is committed to that.
"This is a tough time, there's no doubt about that but we will do everything we can to preserve those jobs."
Analysis
By Huw Thomas, OceanNewsUK business correspondent
In opposition Labour had been accused by the Conservative government of giving false hope to Tata Steel workers, and it’s a charge Sir Keir Starmer continues to resist as Prime Minister.
While it is true that the new Labour government has announced funding for supply chain businesses, this was formed at the cross-party transition board whose remit and budget was set by the previous Westminster administration.
Tata Steel, meanwhile, is pressing ahead with its plans.
It has already invited workers to volunteer for redundancy, and will close the second of two blast furnaces in September.
Since the steelmaker announced its original proposals at the beginning of this year, barely any changes have been made to the overall plan to cut 2,800 jobs and shut the heavy end of Port Talbot’s operation by the autumn.
That’s despite industrial action and political pressure.
With just weeks remaining, the blueprint for Tata Steel’s restructure seems unlikely to change. Any additional investment or job creation is likely to be secondary, and small in scale, compared with what’s being lost.
Eluned Morgan told OceanNewsUK: "We're all very aware that there's a Damocles sword hanging over the Tata situation."
"The situation is difficult, and you know, we have to be prepared for all options."
The first tranche of cash from a £100m fund to help workers and firms affected by restructuring plans was announced last week.
The £13.5m announced was to support local firms whose main customer is Tata Steel find new markets and to 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 UK governments, with the same party now running both.
Asked whether there will be extra cash for the Welsh government to bring down waiting lists, Ms Morgan said: "There's a whole array of issues that we discussed yesterday.
"Because they will be putting some money on the table in England to bring those waiting lists down, we will get some money as a result of that, we'll decide to spend that how we want.
"I'm very keen to make sure that we use that money to bring waiting lists down."
She said the UK government can learn things from the Welsh government. "We've changed the rules, for example, around around GPs and how they should see patients. We've changed their contracts. They haven't done that in England yet," she said.
Ms Morgan added: "We will work together on things that we agree on, but they need to understand that there some things are devolved."
