Missing Argentine submarine may run out of oxygen soon, navy says The submarine was last heard from on Wednesday.

 -- The search for a missing Argentine submarine is entering the "critical" stage, the country's navy said today.

The ARA San Juan, which is carrying 44 crew members on board, was last heard from last Wednesday. Searchers continued today to search an area of over 186,000 square miles in the Atlantic Ocean off Argentina where it is thought the San Juan vanished.

Assuming the ship surface before its last-known communication last Wednesday, tomorrow would mark its seventh day underwater and oxygen will soon be running out, an Argentine navy spokesperson said today.

Weather conditions have been rough in the region, hampering the search. Conditions are expected to improve Wednesday and be excellent for searching. But the window of good weather is narrow: Conditions will again worsen on Thursday.

Four thousand people from seven countries are working around the clock on the search efforts, the navy said.

The submarine went missing while traveling from a base in Ushia, Argentina, on South America’s southern tip, to its home base of Mar del Plata, farther north. It was last heard from about 275 miles off the San Jorge Gulf in southern Argentina, according to the navy.

In Mar del Plata, relatives of the missing sailors congregated and waited for updates on Monday.

Twice, Argentine navy officials have dashed hopes some sign of life may had come from the vessel -- once on Saturday, and then again on Tuesday.

A brother of a machinist on the submarine suddenly interrupted an interview with ABC News today to say he had to tend to the wife of the machinist, Fernando Mendoza.

"I have to run," said the brother, Carlos Mendoza, on Monday. "My sister-in-law just fainted in her room in the base."

Marcela Tagliapetra, a relative of another sailor aboard the submarine, said she felt despair.

“We are waiting for good news so we can have something to celebrate,” she told ABC News Monday. “We are going to get it. We are sure that we are going to get it.”