Rescuers involved in the search for 40 pilot whales still stranded in Florida said they will suspend their operations Sunday after failing to spot the animals via air and sea.

The mysterious mass stranding in a remote area off the US state of Florida's Everglades National Park was first reported on Tuesday. A total of 11 whales have died so far during the ordeal.

The National Oceanic and Atmospheric Administration, which led the search, said Saturday it could no longer see the whales, whose fate has been unknown since Friday.

"We are standing down operations tomorrow (Sunday)," NOAA Fisheries said on its Twitter account.

"If we get reports of pilot whales teams will mobilize ASAP."

The last report from NOAA marine mammal scientist Blair Mase on Friday suggested it was "encouraging" that 24 of the mammals had not been located.

We are "hoping they are out to sea," Mase told reporters in a conference call. But she acknowledged she was "not certain" the missing group had made its way out of difficult, shallow waters to safety.

Of the 11 whales that perished, four had to be euthanized after they became unable to breathe, hydrate and feed in the shallow waters less than three feet (0.9 meters) deep off Highland Beach on the Gulf Coast.

It was still unclear why the whales swam to the warm, shallow waters. The beaching occurred in a very isolated part of southern Florida in Monroe County, where there is no cell phone reception.