Visiting UN disarmament envoy Angela Kane, tasked by Secretary General Ban Ki-moon to establish the terms of an inquiry, struck the accord with the Syrian government for a probe, the foreign ministry said in a statement.
It said a UN team of inspectors who have been in Syria for the past week would be authorised to carry out the investigation in the Damascus region, where the opposition claims regime forces unleashed chemical weapons causing more than 1,300 deaths.
Damascus has strongly denied it carried out an attack using chemical arms, instead saying the rebels are to blame.
The ministry's announcement came as Western pressure grew on President Bashar al-Assad's regime to allow the inspectors access to the site of the August 21 alleged attacks, where Doctors Without Borders said 355 people had died of "neurotoxic" symptoms.
US Defence Secretary Chuck Hagel said the US military was "prepared to exercise whatever option" against Syria but intelligence was still being evaluated.
On a visit to Malaysia, Hagel said the US defence department had prepared "options for all contingencies" at President Barack Obama's request.
Hollande said there was "a body of evidence indicating that the August 21 attack was chemical in nature, and that everything led to the belief that the Syrian regime was responsible for this unspeakable act".
If confirmed, it would be the deadliest use of chemical agents since late Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
Al-Nusra Front, a fierce Al-Qaeda-linked group fighting the regime, vowed revenge against villages of Syrian President Bashar al-Assad's minority Alawite community.