Syrian rebels fighting President Bashar al-Assad should be given anti-tank and anti-aircraft weapons to "level the playing field" in their war and ensure that "extremist" groups do not dominate the opposition, a senior member of the Saudi Arabian royal family has urged.

Prince Turki al-Faisal, a former chief of Saudi intelligence and the brother of the kingdom's current foreign minister, said he was no longer in government and did not need to be diplomatic, but "assumed" weapons were being sent to the rebels. He said it would be a "terrible mistake" if they were not.

"Sixty thousand people have been killed already," Prince Turki said at the World Economic Forum in Davos on Friday. "Do we have to wait for double or triple that number to die before Assad leaves?"

King Abdullah of Saudi Arabia became the first Arab leader to call openly for armed support for the anti-Assad opposition after the uprising began in March 2011. Along with Qatar, Turkey and the UAE, the Saudis are believed to be the rebels' principal suppliers and financiers. But public discussion of the issue is extremely rare and the demarcation between government and private initiatives is blurred.

Turkey's foreign minister, Ahmet Davutoglu, also in Davos, would not be drawn on the question of arms supplies but said there was "no limit to Turkey's assistance to the Syrian people".

In recent weeks alarm bells have been ringing about the growing presence of jihadi elements in Syria's armed opposition. The US, Britain and other western governments are privately urging caution in choosing what types of armaments are delivered, and to whom. Anti-Assad groups are complaining that supplies are drying up. Prince Turki's surprisingly frank remarks appear to reflect that.

"What is needed are sophisticated, high-level weapons that can bring down planes, can take out tanks at a distance," Reuters quoted him as saying. "This is not getting through. You have to level the playing field. Most of the weapons the rebels have come from captured Syrian stocks and defectors bringing their weapons."

Reflecting those concerns, the US recently formally designated the Jabhat al-Nusra group, which is said to have al-Qaida connections, as a terrorist entity.

The Saudi prince said foreign governments should have enough information on the rebel units to ensure that weapons only reached specific groups. "Levelling the plain militarily should go hand in hand with a diplomatic initiative," he suggested. "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."

Earlier this week Turki insisted that the Arab world did not have the capability to handle the crisis alone, clearly implying that western governments should get more closely involved. "It doesn't have the air force, the navy, the army, the intelligence-gathering machinery to go and surgically stop this fighting," he said.

King Abdullah of Jordan, who was also in Davos, also sounded a cautionary note: "The new Taliban we are going to have to deal with will be in Syria." Anyone who thought Assad was going to fall within a few weeks did not understand the complex situation and the balance of forces, he added.

Syrian rebels have seized territory in the north and control suburbs to the east and south of Damascus, but Assad's air power and continued military superiority have limited their advances. More than 60,000 people have been killed in the conflict, which started nearly two years ago with mainly peaceful protests but has now mushroomed into a civil war.