The United States has concluded that the Syrian government used chemical weapons in its fight against opposition forces, and President Obama has authorized direct U.S. military support to the rebels, the White House said Thursday.

Ater the use of chemical weapons against rebels forces, the United States has decided to supply the rebesl with with small arms and ammunition, according to American officials.

The weapons would probably be delivered by air to Turkey or Jordan, or both, and by land into Syria along rebel-held corridors. The opposition’s requests for ­antitank and antiaircraft weaponry are still under discussion.

Some analysts, as well as opposition leaders, have said that rebel militias will need heavier antitank and antiaircraft weapons to push back pro-Assad forces. Administration officials have opposed sending those sorts of weapons, fearing they could easily find their way to terrorist groups. Several of the rebel militias have ties to Al Qaeda and other militant Islamic groups, officials say, and those groups have gained strength as the conflict has dragged on.

Mr. Obama declared last August that the use of chemical weapons by the Syrian government would cross a “red line” that would prompt a more resolute American response.