World must act over Syria chemical weapons - Hollande
However, Mr Hollande cautioned: "We can only act within the framework of international law".
He spoke hours after Syrian government forces retook full control of the strategic western town of Qusair, after a siege lasting more than two weeks.
A BBC News team taken by government forces to Qusair said it was in ruins.
The first Western journalists to reach the city, they said there were few civilians.
Meanwhile, the US and Russia have failed to set a date for proposed peace talks.
The UN and Arab League envoy to Syria, Lakhdar Brahimi, said the international conference might now be held in July, rather than June as had been planned.
He called the lack of agreement between Washington and Moscow "embarrassing", but also noted that neither side in the Syrian conflict was ready to commit to attending.
Sarin 'used'
On Tuesday, French Foreign Minister Laurent Fabius revealed that samples taken from locations of alleged chemical weapon attacks in Syria, including Saraqeb and Jobar, and brought to France had tested positive for the nerve agent, sarin.
Mr Fabius said he had "no doubt" that sarin had been used by "the Syrian regime and its accomplices", but did not specify instances of its use. The US says more proof is needed.
The Syrian government has repeatedly denied using chemical weapons, and has in turn accused the rebels of doing so, an allegation that they have also rejected.
On Wednesday, Mr Hollande told reporters: "We have the elements which now allow us to give certainty over the use of chemical weapons in Syria - at what level we still do not know."
"What has happened in Syria must be one more piece of pressure that can be put on the Syrian regime and its allies."
Qusair 'in ruins'
The town had been the focus of fierce fighting for more than two weeks between rebels and troops backed by a pro-government militia and fighters from the militant Lebanese Shia Islamist group, Hezbollah.
State TV reported that a large number of rebels had died and many others had surrendered as troops advanced swiftly.
The rebels said they withdrew overnight in the face of a massive assault.
"In the face of this huge arsenal and lack supplies and the blatant intervention of Hezbollah... tens of fighters stayed behind and ensured the withdrawal of their comrades along with the civilians," said a statement quoted by the Reuters news agency.
The previously bustling town where 30,000 people once lived is now in ruins, with not one building visible that escaped the intense fighting, reports the BBC's Lyse Doucet, who was taken to Qusair by the Syrian government.
There appears to be only a handful of civilians left - a few farmers on tractors heading out of the town and one family loading up a vehicle, our correspondent says.
She adds that both Syrian troops and Hezbollah fighters are in evidence everywhere - travelling in trucks and armoured vehicles, firing guns in celebration and moving on foot through the streets.
The International Committee of the Red Cross (ICRC) has said it fears there are shortages of food, water and medical supplies.
George Sabra, the interim chairman of the main opposition alliance, the National Coalition, said there were hundreds of injured people awaiting help in the Qusair area and called on the Red Cross to be allowed access.
Qusair lies only 10km (6 miles) from the Lebanese border and is close to important supply routes for both the government and rebels.
The army said Qusair's capture was "a clear message to all those who share in the aggression on Syria... that we will continue our string of victories until we regain every inch of Syrian land".
Correspondents say the battle for Qusair has highlighted Hezbollah's growing role in the Syrian conflict - a development that has heightened sectarian tensions in the wider region.
Hundreds of Syrian refugees have been pouring across the border into the Lebanese town of Arsal, many of them attempting to register with the local municipality for supplies of water, food and other supplies.
More than 80,000 people have been killed in Syria and more than 1.5 million have fled the country since an uprising against Mr Assad began in 2011, according to UN estimates.
The UN reported on Tuesday that the war had reached "new levels of brutality", with evidence of massacres and children being taken hostage of forced to witness - and sometimes participate in - atrocities.
BBC
