"What is needed are sophisticated, high-level weapons that can bring down planes, can take out tanks at a distance. This is not getting through," said Prince Turki al-Faisal, a former intelligence chief and brother of the Saudi foreign minister.
Insurgents in Syria have seized territory in the north of the country and control suburbs to the east and south of the capital, but Assad's air power and continued army strength have limited their advances 22 months into the conflict.
"I'm not in government so I don't have to be diplomatic. I assume we're sending weapons and if we were not sending weapons it would be terrible mistake on our part," the Saudi prince said at the World Economic Forum in Davos, Switzerland.
"You have to level the playing field. Most of the weapons the opposition forces have come from captured Syrian stocks and defectors bringing their weapons," he said.
More than 60,000 people have been killed in the conflict, which started nearly two years ago with mainly peaceful protests but has mushroomed into a civil war that has driven half a million people from the country and displaced many more.
King Abdullah of Jordan, which has taken in some 300,000 Syrian refugees, 20,000 of them in the last week, told the Davos meeting that anyone who thought Syrian President Bashar al-Assad was going to fall within weeks did not understand the complex situation and the balance of forces.
One major problem was that radical al Qaeda forces had established themselves in Syria for the last year and were receiving money and equipment from abroad, he said.
New Taliban in Syria?
Noting that Jordanian forces were still fighting Taliban militants in Afghanistan alongside NATO troops, he said: "The new Taliban we are going to have to deal with will be in Syria."
Even in the most optimistic scenario, it would take at least three years to "clean them up" after the fall of the Assad government, the monarch said.
He called for major powers to craft "a real and inclusive transition plan" for Syria, saying the army must be preserved intact to form the backbone of any new system and avoid the anarchy that prevailed in Iraq after the US-led 2003 invasion.
The United Nations should stockpile food and emergency supplies in Jordan to be moved into areas of Syria controlled by the opposition to prevent more people leaving.
Syria has accused Saudi Arabia, Qatar, Turkey, the United States and France of funding and arming the opposition forces, something they have all denied. But UN diplomats say that weapons are clearly reaching the opposition fighters via Gulf Arab states and Turkey.
Saudi Arabia has called in the past for the opposition fighters to be armed, but diplomats say that Western countries are reluctant to allow sophisticated weapons into the country, fearing they would fall into the hands of increasingly powerful radical forces.
The United States has designated one radical group in Syria - the Nusra Front - as a terrorist organisation and expressed concern about the growing radical militant strength in Syria.
But the Saudi prince said foreign powers should have enough information on the many opposition brigades to ensure weapons only reached specific groups.
"Levelling the plain militarily should go hand in hand with a diplomatic initiative ... You can select the good guys and give them these means and build their credibility," he said.
"Now they don't have the means, and the extremists have the means and are getting the prestige."