"We are analysing more closely to reliably determine that they were not calls coming from the submarine", the admiral added.
The navy has said it believed an electrical outage on the submarine may have caused its communication problems.
There is no sign yet that the problem is anything worse than a communications failure, a spokesman said. "We are trying to squeeze that information to obtain some concrete data and a geolocation".
The Navy is taking both since the location of the submarine is still unknown.
The 2,000-ton, 66 meter long, German-built diesel-electric submersible disappeared Wednesday 430 kilometers off the coast of Argentina.
Following a request for assistance from the Argentine government, the Portsmouth-based navy ship immediately changed course from her planned tasking and sailed to join the multi-national efforts to find the missing submarine ARA San Juan.
Authorities in Argentina are now working on tracing the calls, while an international search mission is underway. "It really complicates the work".
The Navy release explained that the two different systems are being sent because of numerous factors that include varying ocean depths in the area and different safe operating depths of the systems.
ARA San Juan's crew member Eliana Maria Krawczyk is Argentina's first woman submarine officer.
"We analysed these signals, which as we know were intermittent and weak", said Gabriel Galeazzi, a naval commander. "We haven't been able to locate the submarine on the surface".
The aircraft and its 21-person crew are heading for Argentina from El Salvador's Comalapa Air Base, where the P-8A Poseidon was supporting counter-illicit trafficking maritime patrol operations.
Pope Francis, an Argentine, was praying "fervently" for the crew to return home soon, his office said.
The sailors will also be relying on Pressurized Rescue Module, or PRM, which can rescue "up to 16 personnel at a time.by sealing over the submarine's hatch allowing sailors to safely transfer to the recuse chamber", according to officials.
NASA spokeswoman Katherine Brown told CNN a P-3 Orion aircraft, which was "already in Argentina on a scientific mission", would be joining the search efforts.
The search has resonated in the Vatican.
Britain's Royal Navy has deployed the ice patrol ship HMS Protector with Sky reporting that the UK has a C-130 Hercules, in the Falklands Islands, on standby. The Poseidon is armed with torpedoes and cruise missiles.
"We share their concern and that of all Argentines", President Mauricio Macri tweeted, according to Reuters.