At least 13 people were killed when a riverboat sank in the Amazon rainforest region, Brazilian authorities said Monday, as survivors described fleeing the foundering boat in terror.

The ferry was taking about 70 passengers up the Jari River, a tributary of the Amazon, when it suddenly began to tip over Saturday around dawn.

Authorities said they had rescued 46 survivors. It was not clear exactly how many more passengers were missing. Search operations were ongoing, using helicopters, planes and rescue divers.

The Brazilian navy said it had opened an investigation into the accident, whose cause was unknown. Survivor Vanderleia Monteiro said the boat, the Anna Karoline III, seemed to run into trouble when another boat pulled alongside it and tried to anchor, a common practice for the ferries that travel the Amazon and its tributaries.

Someone screamed, "It's sinking!" and within seconds the boat was tipping over, she told Brazilian news site G1, after fleeing with her husband and 11-year-old son. "We escaped through the window and felt our way up the outside wall of the boat as it tipped over. It was like something out of a movie," she said.

"The current swept us downstream fast, and we saw the boat sinking in the distance. Then the other boat rescued us."

The Anna Karoline III, a two-story river ferry, set out Friday afternoon from the city of Macapa, the capital of Amapa state in northeastern Brazil. It was heading for Santarem, in the neighboring state of Para, about a 36-hour trip. Rescue helicopters took about nine hours to arrive because the region is so remote.