Moroccan authorities last month arrested Yassine Abaaoud, a younger brother of the suspected Islamic State mastermind of the Paris attacks after he arrived in his father's hometown of Agadir, a Moroccan security source said.

Abdelhamid Abaaoud, the Belgian militant of Moroccan origin, 28, is believed to have orchestrated last week's coordinated bombings and shootings that killed 129 people. He was among those later killed in a police raid in a Paris suburb.

Yassine was arrested last month after his plane landed in Agadir and has been held in custody since, a Moroccan security source, who declined to be named, told Reuters. "He has been held for about one month now," the source said.

It was unclear if Abdelhamid's younger brother has ties with his militant cell in Europe. The source declined to give further details on the arrest.