Islamic State fighters have made considerable progress near the contested northern Syrian city of Aleppo and are threatening to rout core elements of Western-backed Syrian rebel forces there, France's defence minister said on Wednesday.
The Islamist militant group launched an offensive in northern Aleppo province last month, in a blow to the rebels in an area where the United States and Turkey are planning to open a new front against the movement.
Jean-Yves Le Drian told France Inter radio that one of the reasons why Paris had decided to join air strikes in Syria over the coming weeks was because Islamic State was expanding rapidly.
"What is certain is that things have changed drastically. For several months Daesh (Islamic State) has considerably extended its presence on Syrian territory," he said.
"It's true in Aleppo, but also on the Homs-Damascus line. Today Daesh has progressed in a way that it is threatening the Syrian resistance in Aleppo region, but also Lebanon if it manages to break through the Damascus-Homs axis."
Aleppo was Syria's most populated city and commercial hub before the conflict erupted in 2011. Reduced to rubble in many areas, it has been carved up between government forces and various insurgent groups.
France until now had only taken part in air strikes against Islamic State in neighboring Iraq - contributing just 3 percent of coalition strikes there. It has also provided limited logistical support to rebels, including Kurds, that it considers moderate in Syria.
"The situation is different to last year. Blocked in Iraq, Daesh has been on the offensive in Syria. The situation is very serious," Le Drian said.
Paris was ready to join air strikes on Syrian President Bashar al-Assad's forces in Syria in 2013 before U.S. President Barack Obama backed off the plan.
It has since adopted a more cautious position in Syria and officials have stressed that any air strikes carried out in Syria will be done independently, not under U.S. command.
Le Drian said French air strikes would take place in the coming weeks once targets had been identified.
He reiterated that the only solution to the Syria crisis was through a political transition that would see Assad leave power. When asked if the Syrian leader should leave power immediately he said: "As soon as possible."