"Daesh [an Arabic acronym for IS] is in retreat, that is beyond dispute," Hollande said after a meeting with the leaders of the United States, Germany, Britain, Italy and Ukraine on the sidelines of a Nato summit in Warsaw on Saturday.
But, he went on, "we must also avoid a situation whereby as Daesh becomes weaker, other groups become stronger."
He singled out the Al-Nusra Front, calling for the US, Russia and other allied forces to launch "effective action" against it.
Airstrikes and ground offensives by local forces have meant that IS has lost territory in Syria and Iraq in recent months.
On Wednesday US President Barack Obama and Russia's Vladimir Putin agreed in a telephone conversation to increase military coordination between their countries in Syria.
In May Washington rejected a Russian proposal for joint airstrikes against armed Islamists in Syria.
The White House reported that the two leaders, in their call this week, "confirmed their commitment to defeating ISIL (IS) and the Al-Nusra Front".
The two groups are excluded from a broader truce brokered by Moscow and Washington in February.
Syrian rebels failed bid to reopen their only supply route into Aleppo city overnight, despite fierce fighting in which at least 29 combatants were killed, Syrian Observatory for Human Rights said Sunday.