Opposition forces "will not be victorious no matter how much they conspire" with Washington and its allies in the region, Syrian Foreign Minister Walid al-Moallem told reporters in Turkey on Monday.
Earlier this month, President Obama ordered the CIA to begin setting up distribution points in neighboring Jordan and Turkey to start providing American arms to rebel forces fighting to oust embattled Syrian President Bashar Assad.
The Pentagon has also left 700 U.S. troops, along with a battery of Patriot anti-ballistic missile systems and a F-16 fighter unit, in Jordan, to help Amman protect its border with Syria.
The American weapons and troops were part of the recently-completed U.S.-Jordanian military exercise, dubbed Eager Lion.
The decision to arm the rebels came after U.S. intelligence officials concluded the Assad regime had used chemical weapons against opposition forces.
Assad's use of those weapons crossed a "red line" with the Obama administration, prompting the president to approve the arms supplies to Syria's rebels.
Al-Moallem's comments come on the eve of a multinational peace summit in Geneva to end the civil war and days after Secretary of State John Kerry met with opposition leaders in Doha, Qatar.
Gen. Salim Idris, the top commander of the Free Syrian Army (FSA), said last Friday his forces had recently obtained the weapons but refused to say who supplied the arms.
"I would like to thank the brothers and the friends whom I don't want to name," he said in an interview with Al-Jazeera on Friday.
The FSA is the largest and most organized of the rebel factions battling to overthrow Assad's forces in the country.
On Monday, al-Moallem was defiant in his response to the additional American aid to rebel forces.
"We are not scared," he said of the American arms shipments.
"If [opposition forces] dream or are delusional about achieving a balance with the Syrian Arab Army, I think they need to wait years and this won't be achieved."
That said, the Assad regime is not entertaining any option of relinquishing control of the country, despite Assad's participation in the upcoming Geneva talks, according to al-Moallem.
"President Bashar Assad will not step down," he said. "If anyone has such illusions on the other side, my advice to them is not to go to Geneva," he said.