Argentina's navy says it has lost contact with a submarine off the southern coast - but does not consider it lost.

The navy's Twitter feed said communications were lost on Wednesday with the ARA San Juan, a German-built diesel-electric vessel with 44 crew aboard.

It said navy ships and aircraft are searching near the last known location of the sub off the province of Chubut. It also said officials are scanning all possible radio transmission frequencies for a sign of the San Juan.

Navy spokesman Enrique Balbi told the Todo Noticias channel there was no indication of problems from the submarine and said it could not yet be termed as lost.

"It's not that it's lost, because to be lost, you have to look for it and not find it," he said.

Admiral Gabriel Gonzalez, chief of the Mar del Plata base that was the submarine's destination, said the sub had sufficient food and oxygen.

"We have a loss of communications, we are not talking of an emergency," he said.

Relatives of some of the crew members are at the base awaiting word of the search.

"We are praying to God and asking that all Argentines help us to pray that they keep navigating and that they can be found," said Claudio Rodriguez, brother of one of the crew members.

"We have faith that it's only a loss of communications."

Mr Balbi said the sub was headed from the naval base at Ushuaia in the country's extreme south to Mar del Plata, about 250 miles south of Buenos Aires.