The collapse of a deal for the oil refinery in Milford Haven is "very disappointing", David Cameron has said.
But the Prime Minister insisted ties should not be cut with Klesch Group, which is also in the process of buying Tata Steel's long products division.
Mr Cameron made the remarks in answer to Labour MP Tom Greatrex who highlighted concern about the Swiss-based firm's intentions given its record of "asset stripping".
Speaking at Prime Minister's Questions, Mr Cameron said, "First of all, let me agree with you it is very disappointing what has happened at Milford Haven and we will continue to work with the company concerned and try to find employment opportunities for all those that work there.
Union representatives will be holding urgent meetings with Welsh Government ministers later today and will press the Westminster to intervene to secure an alternative buyer.
Britain’s largest union, Unite is calling for a taskforce originally set up to secure the future of Murco’s Milford Haven oil refinery to be reconvened following the collapse of the refinery’s sale. The refinery employs over 400 people and its closure could undermine the future security of fuel supply, warns the union.
The workforce has shown great professionalism and resilience in these deeply uncertain times and need to be assured that every avenue will be explored to ensure there is a future for oil refining at Milford Haven.
A deal aimed at securing the future of an oil refinery has collapsed, threatening hundreds of job losses. Welsh and UK Government ministers expressed their disappointment that an agreement to safeguard the future of the Murco refinery at Milford Haven in West Wales could not be achieved.
Pembrokeshire County Council says it is liaising with other agencies, such as Job Centre Plus and Pembrokeshire College in order to offer as much assistance as possible to the refinery employees and the supply chain companies.
Council Leader, Council Jamie Adams, said the announcement by Murphy Oil was “devastating news” for the workforce and their families, especially as the sale of the refinery seemed to have been agreed earlier this summer.
“We will be doing everything we can to soften the blow for the individuals affected,” he said.
US firm Murphy Oil Corporation said its wholly owned subsidiary, Murco Petroleum Limited, was unable to complete the sale of the Milford Haven refinery and terminal assets to Klesch Refining Limited by a deadline of October 31.
"The refinery is currently in a period of shut-down, and will be decommissioned and operated solely as a petroleum storage and distribution terminal. The company will seek a buyer for the terminal facility along with three inland terminals. The sale of the company's retail assets is unaffected by the inability to sell the refinery," it said a statement.
Economy minister Edwina Hart said the work will not stop to help find a buyer for the Murco site.
Her comments come as the Welsh and UK Government announced an agreement by Swiss-based Klesch Refinery Ltd to buy the site had collapsed, leaving hundreds of jobs at risk.
“This is a devastating blow to staff at the Murco refinery and companies within the refinery’s supply chain, as well as the local communities around Milford Haven. “Our support for the Murco refinery and its staff will not stop here. We have been working with management at Murco since 2010 to try to maximise the opportunities for the sale of the refinery.
“We will all continue to work tirelessly with Murco management to maximise the opportunities for employment at the site, ensure the ongoing viability of the terminal and assist to find a new owner.
The Welsh Secretary has expressed his disappointment at the news the deal to buy Milford Haven oil refinery Murco has collapsed.
Stephen Crabb took to Twitter to say he was 'desperately sorry' for Murco staff, whose jobs are now under threat yet again.
Incredibly disappointing news about Murco refinery tonight. Desperately sorry for workforce & their families - endured months of uncertainty
Grateful to UK & Welsh Govs for all their efforts to keep Murco refinery deal alive.The closure will be a huge blow to West Wales economy
Hundreds of jobs at the Murco refinery in Milford Haven are at risk tonight after a deal to buy the site collapsed.
Both the Welsh and UK Government has expressed its disappointment at the news.
400 jobs looked like they were safeguarded when Klesch Refining Ltd agreed a sale to buy the site back in the summer.
The deal was approved by the European Commission in October.
Wales Office minister Baroness Randerson said:
"This is extremely disappointing news for the workforce at Murco and their families.
“Supporting the staff at Murco is now our immediate concern and we stand ready to assist in any way we can."
Police investigating an alleged forgery in Milford Haven want to speak to this woman.
A £50 note was used to purchase low value items and change given. The note was later found to be forged.
Police believe the woman in the CCTV footage may be able to help with their investigations.
She is described as wearing a white top and black/white patterned trousers.
Anyone who can identify her or provide information is asked to contact PC Anthony Coleman at Milford Haven Police Station on 101.
Around 400 jobs have been saved after an agreement over the sale of the Murco oil refinery in Milford Haven.
The deal has been welcomed by Pembrokeshire Council, who have described the sale as 'good news' for Pembrokeshire and Wales.
The news has also been welcomed by the Minister for Economy, Science and Transport. In a statement today, Edwina Hart AM said: "Today’s announcement is very good news and the first step in securing the long-term future of the Milford Haven refinery."
The Secretary of State for Wales has welcomed the news that a deal has been reached over the future of the Murco oil refinery in Milford Haven.
On twitter, the Wales Minister described the deal between Murco Petroleum Ltd and Klesch Refining Ltd as 'excellent news' for Murco employees.
He added that it is "important to maintain UK oil refining".