Fabius said if reports by opposition activist groups are correct, that more than 1,300 people died in a chemical weapons attack outside Damascus, "a reaction of force must be taken," CNN reported.
An emergency session of the U.N. Security Council was called after the report emerged, but Russia and China blocked a formal resolution.
Fabius said after the session, if the U.N. Security Council "cannot do it, decisions will be made otherwise." However, he ruled out sending ground troops into Syria.
Speaking in Berlin, Turkish Foreign Minister Ahmet Davutoglu called on the United Nations to act decisively, saying "all red lines" had been crossed, referencing a phrase coined a year ago by U.S. President Barack Obama.
The Security Council "can't assume an undecisive attitude," he said.
Davutoglu called on Syria to allow U.N. inspectors to visit the site of the alleged bombing immediately.
A U.S. official said the White House has "strong indications" the Assad regime used nerve gas in attacks on Syrian rebels.
Israeli Minister of Strategic Affairs Yuval Steinetz backed up the U.S. claim.
"According to our intelligence assessments there was use of chemical weapons, and this of course was not for the first time," The New York Times reported.
He called the U.N. investigation "a joke," CNN said.
Syrian authorities denied using chemical weapons and accused the opposition of fabricating claims or staging gas attacks.
Russia alleged rebels staged the attack to get the U.N. Security Council to side with the opposition and scuttle possible peace talks.
At least five towns were gassed in the pre-dawn attacks in Damascus suburbs, where rebels have had recent success in fighting off regime forces, opposition groups and activists said.
Dozens of videos posted online showed scores of lifeless bodies sprawled on floors. Some men were wrapped in burial shrouds, some children were in diapers. No wounds were visible.
Other videos showed children on makeshift hospital floors vomiting, convulsing and struggling to breathe. Some were being treated with hand-held respirators while others received cardiopulmonary resuscitation.
Men sprawled on the floor were shown being hosed down.
Women were killed and treated, but none were recorded on video out of respect, opposition activists said.
The Western-backed Syrian Opposition Coalition said more than 1,100 people died. The Local Coordination Committees of Syria, a network of opposition activists, said at least 1,360 were killed. The United Medical Office of al-Ghouta, an umbrella group for opposition-run field hospitals, put the number of dead at 1,600.
The alleged attack sites weren't accessible to journalists, so it was not immediately possible to confirm the allegations or casualty counts.
A senior Obama administration official told The Wall Street Journal late Wednesday there are "strong indications there was a chemical weapons attack -- clearly by the government."
The indications are preliminary and "we do need to do our due diligence and get all the facts and determine what steps need to be taken," the official said.
During a daily briefing with reporters Thursday, White House spokesman Josh Earnest said the United States is "appalled" by reports of "widespread civilian killings."
Earnest said senior Obama administration officials have been in contact with their counterparts worldwide to coordinate a response. There was some consultation at the U.N. Security Council Wednesday as well, he said.
"But what is true today is something that was true yesterday, which is that there is a U.N. chemical weapons investigative team on the ground in Syria right now," Earnest said. "You have an Assad regime that denies responsibility for the use of these chemical weapons. The easiest way for them to demonstrate that they are on the side of the international community in opposition to the use of chemical weapons is to allow this U.N. team full access to the site to try to get to the bottom of what happened."
He said the U.N. team must be allowed to interview witnesses, collect physical samples and have "unfettered access to the region so that they can do their work," Earnest said.
"And we renew our call for the Assad regime to do exactly that," the White House spokesman said.
The United States, the European Union and other world powers called for the 13 U.N. weapons inspectors in Syria investigating other sites of alleged chemical-weapons attacks to immediately visit Wednesday's attack sites.
The team is based 5-to-10 miles away.
Russia, a regime supporter, suggested rebels had launched a chemical attack and blamed it on the regime to win U.N. support and thwart a planned peace conference in Geneva, Switzerland.
"All of this really looks like an attempt, at any cost, to create a reason to produce demands for the U.N. Security Council to side with the regime's opponents and undermine the chances of convening the Geneva conference," Foreign Ministry spokesman Alexander Lukashevich said in a statement.
The Security Council, meeting in emergency session, called for a prompt investigation of the allegations and appealed for a cease-fire, but took no further action.
British Deputy U.N. Ambassador Philip Parham said on Twitter late Wednesday 37 countries had signed a letter to Secretary-General Ban Ki-moon formally requesting Syrian authorities grant the U.N. investigators "urgent access to all relevant sites and information sources.