Defence Secretary Gavin Williamson has offered Britain's "sincerest condolences" to Argentina amid fears that all 44 crew members on a missing submarine have died.
The Argentine navy has said that a sound thought to be an explosion had been detected by US and specialist agencies leading the search.
The "hydro-acoustic anomaly" was produced just hours after the navy lost contact with the ARA San Juan on November 15, said Enrique Balbi, a spokesman for the Argentine navy
Balbi said the search will continue until they know for certain what happened to the submarine, but he said evidence showed "an anomalous event that was singular, short, violent and non-nuclear that was consistent with an explosion."
A fleet of international vessels and aircraft joined the search for the San Juan, including the RAF Voyager, the first British military aircraft to land in Argentina since the Falklands War.
"This has not just been an agonising time for Argentina, but for our whole international community, and this news is truly devastating for everyone involved in this week's search and rescue operation.,” Williamson said.
"When it comes to situations like this, nationality doesn't matter: all sailors have an obligation to help each other in a time of such desperate need.
"I'm extremely proud of the role our Navy and Air Force have played in the international effort to help, and I speak on behalf of those involved, and the whole country, in offering our sincerest condolences to Argentina and the family and friends of the crew."