PARIS (Reuters) - A man and his pregnant, teenage girlfriend suspected of plotting to kill in the name of the Islamic State in Iraq and Syria (ISIS) were put under formal investigation on Friday (Oct 14) following their arrest on Tuesday in the Paris suburb of Noisy-le-Sec, a judicial source told Reuters.

The two partners, aged 21 and 17, were in contact with ISIS militant Rachid Kassim via the Telegram encrypted messaging system, the source said, confirming an earlier report by French newschannel iTele.

Kassim is suspected of having participated in several attacks or attempted attacks in France.

French police arrested at least six teenagers last month, all suspected of having plotted to kill in the name of ISIS, after having been spotted on social networks.

All were in contact via Telegram with Kassim, who is currently located in the Syrian-Iraqi region, according to police and judicial sources.

France is reeling from a wave of militant attacks that have killed more than 230 people since January 2015, and its intelligence services are struggling to dismantle a web of militant networks inside the country.

In November last year, a squad of suicide bombers and gunmen killed 130 people in a coordinated attack on multiple sites in Paris.

In July, a Tunisian man killed 86 people when he drove his truck through a Bastille Day crowd on Nice's seafront. Later that month, two militants knifed to death an elderly priest at his altar in a church in northern France.