Argentina on Saturday declared three days of national mourning after an Argentinian Navy submarine was found, one year and a day after it went missing.

Argentinian President Mauricio Macri confirmed that the submarine had been located and promised "serious investigations" in a bid to find out what happened to the vessel.

The ARA San Juan was found was found at a depth of 907 metres in the South Atlantic, about 500 kilometres east of Argentina's coast.

US survey firm Ocean Infinity said it found the ship near where it had last been reported on November 15, 2017.

"Now a stage of serious investigations is opening to know the truth, it is necessary to honor and respect our heroes and relatives, to whom I wish all the strength to overcome this difficult moment," Macri said in a message to Argentinians.

A massive international search was launched after the submarine disappeared last year, but hopes of finding the crew of 44 alive were given up two weeks later and the search was scaled down.

On Thursday, Ocean Infinity was just 48 hours away from giving up its search when it found an unknown object on the ocean floor. Closer inspection using a remote submarine revealed it to be the missing vessel.