Argentina's navy was hunting Friday for one of its submarines, which has been reported missing in the South Atlantic with a crew of 44 on board.
The navy said it had not had any contact with the submarine, the San Juan, for 48 hours.
"We have not been able to find, or have visual or radar communication with the submarine," navy spokesman Enrique Balbi told a news conference.
The United States, Britain and Chile had offered "logistical support and information exchange in this humanitarian search," the foreign ministry in Buenos Aires said.
The TR-1700 class diesel electric submarine had been returning from a routine mission to Ushuaia near the southernmost tip of South America, to its base at Mar del Plata, around 400 kilometers (240 miles) south of Buenos Aires.
The San Juan's last contact with the navy command was on Wednesday morning, Balbi said.
Argentina said it had launched an air and sea search on Thursday, involving a destroyer and two corvettes.
An initial search in an area around the sub's last known position, some 430 kilometers off the southeastern Valdez peninsula, provided no clues.
Balbi said an initial search was hampered "because it was carried out at night and in bad meteorological conditions prevailing in the area of operations."
The three navy ships and two aircraft flying rotations had "already swept 15 percent of the search area," Balbi told reporters.
The vessel had not activated its emergency beacon, he said.
The navy denied a press report that there may have been a fire onboard.
Balbi appealed for caution.
"I don't want to dramatize the issue. We're lacking communication and don't know what happened," he said.
"There may be a battery issue, a problem of power supply," the spokesman said, adding that navy protocol was that the submarine would surface if any power problems were detected.
The San Juan sailed 10 days ago from Mar del Plata to Ushuaia. It spent three days there before heading back to base, Balbi said.
Among those on board is Argentina's first female submarine officer, 35-year-old weapons officer Eliana Krawczyk.
"Let us pray that nothing has happened to any crew member. At sea they are all brothers, and a submarine carries more risk than a ship," her father Eduardo told Todo Noticias TV.
The San Juan is one of three submarines in the Argentine fleet.
Sixty-five meters (213 feet) long and seven meters wide, it was built by Germany's Thyssen Nordseewerke and launched in 1983.
It underwent a re-fit between 2007 and 2014 to extend its usefulness by some 30 years.