WASHINGTON — The Obama administration, concluding that the troops of President Bashar al-Assad of Syria have used chemical weapons against rebel forces in his country’s civil war, has decided to begin supplying the rebels for the first time with small arms and ammunition, according to American officials.

The officials held out the possibility that the assistance, coordinated by the Central Intelligence Agency, could include antitank weapons, but they said that for now supplying the antiaircraft weapons that rebel commanders have said they sorely need is not under consideration.

Supplying weapons to the rebels has been a long-sought goal of advocates of a more aggressive American response to the Syrian civil war. A proposal made last year by David H. Petraeus, then the director of the C.I.A., and backed by the State Department and the Pentagon to supply weapons was rejected by the White House because of President Obama’s deep reluctance to be drawn into another war in the Middle East.

But even with the decision to supply lethal aid, the Obama administration remains deeply divided about whether to take more forceful action to try to quell the fighting, which has killed more than 90,000 people over more than two years. Many in the American government believe that the military balance has tilted so far against the rebels in recent months that American shipments of arms to select groups may be too little, too late.