Officials said four suspects were arrested on Sunday in the Paris area while a fifth was detained on Monday in the southern port of Marseille.
One of the five suspects, Moroccan national Said al-Maghrabi, had been sought for months by several police forces across Europe.
Al-Maghrabi was placed on Thursday under judicial investigation, a first step towards possible formal charges, for criminal association as part of a plot.
The five were arrested as part of an anti-terrorist investigation launched last September after evidence emerged of a so-called Iraqi network recruiting volunteers to fight US forces.
The Moroccan allegedly spent time in Afghan training camps run by Usama bin Ladin's al-Qaida network. Upon his return, he tried in vain to go to Chechnya where separatist rebels are battling Russian forces.
"He has an active past," a source said.
Al-Maghrabi was preparing to leave for Iraq, via Syrian capital Damascus, when he was apprehended on Sunday.
In January, 11 suspected Islamists were taken into custody including 23-year-old Farid Binyattu, who along with two others has been placed under investigation for "criminal association related to a terrorist enterprise".
French police believe Binyattou was the mastermind behind the recruitment operation, which allegedly sent young men to wage war against US-led forces in Iraq.