Argentina's navy lost contact with one of its submarines and its 44 crew members, it announced on Friday.

Although the German-built ARA San Juan has been out of contact for two days, the navy initially did not consider it lost. "It's not that it's lost, because to be lost, you have to look for it and not find it," the navy told a local broadcaster.

However, after an initial search operation in the submarine's last known position, around 430 kilometers (267miles) off the southeastern Valdez peninsula, bore no results, Argentina said it was stepping up its search efforts and calling on allies for help. "We have not been able to find, or have visual or radar communication with the submarine," navy spokesman Enrique Balbi told reporters at a news conference. "I don't want to dramatize the issue. We're lacking communication and don't know what happened."

The incident has formally been upgraded to a "search and rescue" operation, Balbi added.

Pope Francis has announced Saturday that he is dedicating "fervent prayers" to the 44 submarine crew, and extended his sympathy to the their families.

Where it could be: It last phoned home off Chabut on Argentina's south coast. That's where authorities are searching.

What went wrong: According to navy spokesman Balbi, the working hypothesis is that the vessel suffered an electrical outage, which would explain its current communications difficulties.

What it could be doing: Navy protocol in such cases is to surface and keep on going towards its destination. Bad weather has hampered visual search efforts.

Which countries are helping in the search: A number of countries have offered their assistance in uncovering the lost submarine. Argentina has already accepted an offer from the US to use a NASA P-3 explorer aircraft, which was already stationed in the southern city of Ushuaia, preparing to depart for Antarctica. Brazil, Uruguay, Chile, Peru, Britain and South Africa have also all offered assistance.

ARA San Juan was en route back to its base in the city of Mar del Plata in the Buenos Aires province when it went missing. It had just completed a routine mission in Ushuaia, a coastal town near South America's southernmost tip.

Other stories about submarines you may be interested in:

- German WWI submarine discovered off Belgian coast

- Danish police find head and legs of Swedish journalist Kim Wall

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cl,aw,dm/rt (AP, AFP, Reuters)