A multinational search intensified on Tuesday for Argentina’s missing submarine with 44 crew members off the country’s coast, as the clock may be ticking on the crew’s oxygen supply.

The German-built San Juan submarine has been missing since Wednesday, when it reported a battery failure and lost contact with the Argentine Navy. Two Argentine Navy ships detected signals a day ago from about 220 miles off the coast at a depth of 650 feet, but the source of the signals could not be determined, the BBC reported.

“Some sources were saying that this was banging on the hull in Morse code signals,” Enrique Balbi, a navy spokesman, said. He said the sources could not be verified.

The submarine reportedly carried enough food, oxygen and fuel for the crew to survive about 90 days on the sea’s surface, but only enough oxygen to last seven days underwater, he said.

The submarine was originally scheduled to arrive Sunday in Mar del Plata, a navy base about 250 miles southeast of Buenos Aires.

A U.S. Navy P-8 Poseidon aircraft assisted in the search efforts by deploying its first aircraft on November 18 and a second ship with special tracking equipment and deep-sea rescue modules on November 20, the US Air Force stated.

Argentine President Mauricio Macri met with family members at the base as they waited for news about their loved ones. Concerned family members have taken to social media to plead for help in the search process.

“Pray so that my husband, Fernando Santilli, can return home,” Jessica Gopar wrote on Twitter. “He’s in the San Juan submarine."

More than a dozen international vessels and aircraft have joined the search for the missing submarine, which has been set back by stormy weather that has caused waves up to 20 feet. Eleven boats and 10 planes from Argentina, Brazil, Chile, Peru, South Africa, Uruguay and the UK were assisting in the search as well, Balbi said.