WASHINGTON — The Syrian opposition will not attend the proposed Geneva conference on the crisis in Syria unless rebel fighters receive new supplies of arms and ammunition, the top rebel military commander said Friday.

“If we don’t receive ammunition and weapons to change the position on the ground, to change the balance on the ground, very frankly I can say we will not go to Geneva,” Gen. Salim Idris said in a telephone interview from his headquarters in northern Syria. “There will be no Geneva.”

Secretary of State John Kerry announced in May that the United States and Russia planned to organize an international meeting that would bring together representatives of President Bashar al-Assad’s government and the Syrian opposition. The aim of the meeting, which has emerged as the centerpiece of Mr. Kerry’s Syria strategy, is to negotiate a transitional government that would take control if Mr. Assad agreed to vacate his position. A date for the Geneva conference has yet to be set.

But since that announcement, Mr. Assad’s military position has been strengthened by flights of arms from Iran and the involvement of thousands of fighters from Hezbollah, the Lebanese militant group. The change of fortunes on the battlefield was illustrated last week when the Syrian military and Hezbollah fighters captured the town of Qusayr.