Three former employees with cement giant Lafarge have been interviewed by French investigators about claims the firm indirectly paid armed groups, including the Islamic State (IS), to keep a plant running in a war zone, a source said Wednesday.

The three former workers -- a computer expert, an engineer and an employee who worked in the plant's packaging section -- came from Syria to be interviewed by examining magistrates in Paris on Monday, Tuesday and Wednesday.

"They confirmed the overwhelming nature of the allegations that may be held against Lafarge," the trio's attorney, Marie Dose, told AFP.

They are the first witnesses to be heard in a judicial inquiry launched in June that centres on alleged funding of a terrorist group and on endangering lives.

The probe focuses on claims that in 2013-14, the then French-based cement giant channelled payments to IS in order to continue operations at a plant at Jalabiya in war-torn Syria.

The probe also focuses on whether managers in France were aware of the alleged practice and of the risks this would have presented for its employees in Syria.

The case came to light in an investigation in Le Monde in June 2016, which reported on "troubling arrangements" between Lafarge Cement Syria and IS, which was then on the ascendant.

In March, Lafarge admitted that the group's Syrian subsidiary had indirectly paid protection money to "armed groups, including sanctioned parties, in order to maintain operations and ensure safe passage of employees and supplies to and from the plant".

The alleged dealings took place during 2013 when "the deterioration of the political situation in Syria posed very difficult challenges for the security and operations of the plant and its employees", it said.

Lafarge bought the factory in 2007 and invested some $680 million (572 million euros) to get it working by 2010, representing the biggest foreign investment in the country outside the petroleum sector.

The plant was finally evacuated in 2014 and closed down before Lafarge merged with its Swiss competitor Holcim in 2015.