There has been no contact with the ARA San Juan since Wednesday

A multinational armada of aircraft and vessels battled high winds and giant waves Sunday as they intensified their search for a missing Argentine submarine, after attempted distress calls raised hopes the 44 crew members may still be alive.

There has been no contact with the ARA San Juan, a German-built diesel-electric sub, since early Wednesday.

An air and sea search is underway with help from countries including Brazil, Britain, Chile, Uruguay and the United States.

Hopes of finding survivors were revived when the navy said Saturday that its bases had received seven satellite calls attributed to the submersible.

The signals were received at 10:52 am (1352 GMT) and 3:42 pm (1842 GMT), but they did not lock in, thus preventing a full connection.

"The effort has tripled," Argentina's navy spokesman Enrique Balbi told reporters Sunday of rescue efforts, adding that no theory had yet been ruled out at to what happened to the vessel.

- Seven-meter waves -

The calls revived hopes that the submarine has surfaced, but a powerful storm that has whipped up waves reaching seven meters (23 feet) in height has made geolocation difficult, officials said.

The last regular communication with the San Juan was at 1030 GMT on Wednesday, when the submarine was 430 kilometers (265 miles) off Argentina's coast in the Gulf of San Jorge.

Rescuers are focusing on an ocean patch about 300 kilometers in diameter, radiating from the last point of contact.

The US Southern Command said Sunday that it was sending a second Navy P-8A Poseidon aircraft to join the search. The Florida-based plane and a crew of 21 are to reach Argentina later in the day.

A NASA P-3 research aircraft is already participating in the search, the Southern Command said.

The California-based Undersea Rescue Command also said it was deploying two underwater crafts designed to rescue trapped submarine sailors at different depths, as well as a remotely operated underwater robot known as an ROV.

- 'Got to be afloat' -

"We will do what is necessary to find the submarine as soon as possible," Argentine President Mauricio Macri wrote on Twitter.

Among those on board is Argentina's first female submarine officer, 35-year-old weapons officer Eliana Krawczyk

All land communications bases along the coast were ordered to scan for any follow-up signals, as family members of the missing waited nervously in the coastal city of Mar del Plata.

Claudio Rodriguez, whose brother Hernan is aboard the submarine, was hopeful, saying the satellite signals suggested the vessel was still afloat and would be found.

"They've got to be afloat. Thank God," he said.

"That gives us hope, because we knew that if they were down below, they would be screwed," he told TN news from the western city of Mendoza.

Among those on board is Argentina's first female submarine officer, 35-year-old weapons officer Eliana Krawczyk.

The navy has not ruled out any hypothesis, a spokesman said. The most likely scenario given is that an electrical problem may have unexpectedly cut off the vessel's communications.