An American family of four who were missing since Sunday (29 October) after river pirates attacked a boat they were aboard in the Amazon River delta area in Brazil, have been found, authorities said.

The California couple and their two daughters had been traveling through South America when they went missing. Authorities identified them as Adam Harris Harteau, 39, Emily Faith Harteau, 36, 6-year-old Colette Harteau and 3-year-old Sierra Harteau.

According to The Sun, the family was robbed by pirates around 7pm on Sunday, while traveling on a ferry to Breves, a town in the Amazon in the northern state of Pará.

"Several heavily armed men boarded the ferry and pusher and kept the passengers and crew hostage for hours," a police spokesman said. "The family were in a van and we found it abandoned in the forest but no sign of the couple and their young children."

Some reports claimed the family had jumped into the river after being held hostage for hours.

Investigators reportedly attempted to contact the family's relatives in the US and checked social media to see if they had been in touch with anyone.

The Wall Street Journal reported that a family spokesman had spoken to the US Embassy in Brasilia and was told the family was found in relatively good health.

Officials said the family had recently been living in the southern Brazil city of Florianopolis and had a blog dedicated to their travels, The Associated Press reported. After being found, the family was being taken to Breves for medical examinations.