ANKARA (Reuters) - The United States is studying setting up a limited no-fly zone in Syria close to the southern border with Jordan, two senior Western diplomats in Turkey said on Friday.

Their comments, confirmed by a third regional diplomat, came after Washington said it would step up military assistance to rebels battling President Bashar al-Assad in response to what it said was proof of chemical weapons use by Assad's forces.

"Washington is considering a no-fly zone to help Assad's opponents," one diplomat said. He said it would be limited "time-wise and area-wise, possibly near the Jordanian border", without giving details.

Syrian rebel commanders were due to meet Western and Turkish officials in Turkey on Friday to discuss military assistance to the rebels. Until now Washington has been deeply reluctant to send weapons, citing a risk that they would end up in the hands of radical Sunni Muslim brigades.

One of the diplomats said that setting up a no-fly zone might help Western efforts to monitor the recipients of any arms supplies, as well as helping train the anti-Assad fighters.

(Reporting by Parisa Hafezi; Editing by Dominic Evans)