Two couples were caned in Indonesia's staunchly Muslim Aceh province on Friday after being caught having extramarital affairs, a prosecutor said.

About two hundred people gathered after Friday prayers to watch the four people receive their allotted lashes at a mosque in the city of Jantho, southeast of the provincial capital, Banda Aceh.

Sudirman, 43, and Irdayanti, 34, received nine lashes each.

"The reason for the harsh punishment is that both of them are already married to other people," prosecutor Bendry Almy said.

The other couple punished were Rudi Setia Yudda, 40, who received seven lashes, and 18-year old Nuramalia, who received four.

Irdayanti, who is married to another man, cried as the punishment was carried out and afterwards fainted. She was treated at a local hospital.

The crowd shouted at Sudirman and Yudda, saying their adulterous behaviour had caused them to be flogged and asking that they be meted out extra lashes.

Aceh, on the northern tip of Sumatra island, is one of several areas in Muslim-majority Indonesia where Islamic sharia law has been adopted. Caning as carried out there is mainly aimed at causing shame rather than injury.

In December, two people were caned in Aceh after being caught having an extramarital kiss.