"The US is distributing non-lethal aid and ... some lethal assistance as well to the SMC (Supreme Military Council)," Khalid Saleh told a news conference on Tuesday, referring to the council that oversees operations of militants loyal to General Salim Idriss.
White House officials suggested in June that President Barack Obama had decided to provide military aid to the Syrian militants, but in the months since, rebel leaders and US lawmakers have said no lethal assistance has arrived.
The United States is providing lethal assistance "because they are sure that the mechanisms that the SMC has established are well tested and they will be sure that the weapons are not falling into the wrong hands," Saleh said.
He apparently referred to Washington's concerns that US arms could end up benefiting other terrorist groups, such as the al-Nusra Front, active in northern Syria.
Saleh's comments at a Washington news conference may be the first public indication that US-provided military goods such as arms or ammunition are actually moving to anti-Assad forces.
Secretary of State John Kerry said on Tuesday that Washington was trying to upgrade its support for the Syrian opposition.
"It is ramping up, but I can tell you that many of the items that people have complained were not getting (to) them are now getting to them," Kerry said in a Google+ Hangout interview. He declined to say what military items were arriving.
Saleh also said rebel military leaders were coordinating with the countries that might participate in a US-led strike on Syria.
He said the Supreme Military Council also had a plan to derive tactical benefits from the strikes if they do take place, such as by securing areas that are hit.
The chances of US military action have receded, however, since Russia offered a proposal on Monday for Syria to surrender its chemical weapons stockpile to international control.
After two years of balking at directly arming the Syrian opposition, the White House's pledge in June to provide military aid to the militants, came as the US government claimed it had proof that Syrian forces had engaged in small-scale use of chemical weapons earlier this year.
Rebel sources recently confirmed receipt of arms such as anti-tank weapons financed by Saudi Arabia, which arrived last month through Jordan.
However, Republican and Democratic lawmakers on congressional intelligence committees held up the covert US effort for a while over concerns that the arms could end up in the hands of terrorists in Syria. In late July, lawmakers gave the plan the green light.
Last week, a former senior US official said a limited quantity of American arms had reached the militants.
In the Google+ interview, Kerry said, "It is accurate to say that some things have not been getting to the opposition as rapidly as one would have hoped."
The CIA, which oversees the covert arming effort, had no comment on Saleh's remarks.
Lawmakers who favor arming the rebels, such as Republican Senators John McCain and Lindsey Graham and Democrat Carl Levin, have urged Obama to step up US support for the opposition as part of any plan to take military action in Syria.
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