The oil-rich Gulf monarchies have sided with the opposition from the start of Syria's conflict in March 2011, with Riyadh leading calls for the fall of Assad.
Syrian Deputy Foreign Minister Faisal Muqdad told AFP this week that Saudi Arabia was providing unfettered support for "terrorist groups" in Syria, while other nations had reviewed their positions.
"I think that all those who supported these terrorist groups have the feeling now that they have made big mistakes," Muqdad said in an interview on Thursday, referring to the rebels seeking to topple Assad.
"The only party who is declaring the full support to the terrorist groups, to Al-Qaeda, is Saudi Arabia," he said.
Muqdad urged the world to press Saudi Arabia to halt its support for the rebels, to prevent what he said was "another 11 September incident".
"I think that if the world wants to avoid another 11 September incident, they must start telling Saudi Arabia 'enough is enough'," he said, referring to Al-Qaeda's 2001 attacks on the US.
Earlier this month, Assad's government urged the United Nations to take a stand against Saudi support for Islamist groups whose influence has grown on the battlefield.
"We call on the UN Security Council to take the necessary measures to put an end to the unprecedented actions of the Saudi regime, which is supporting takfiri (Sunni extremist) terrorism tied to Al-Qaeda," the foreign ministry said in a message to UN chief Ban Ki-moon.
It was the first time the Syrian government has appealed to the international body to take action against Riyadh.
"Saudi Arabia is not content to merely send weapons and to finance but also mobilises extremist terrorists and sends them to kill the Syrian people," the Syrian message said.
Saudi 'not to stand idle'
Saudi-Syrian relations had been tense for years, long before the start of the brutal conflict that has now killed an estimated 126,000 people.
The Sunni-ruled kingdom severed diplomatic relations with Damascus following the February 2005 assassination in Beirut of Lebanese ex-premier Rafiq Hariri who had close ties with Riyadh.
Four years later, diplomatic ties resumed and Assad, who belongs to the Alawite Shiite sect, paid an official visit to Riyadh in March 2009.
Saudi King Abdullah, who rarely embarks on official visits abroad, reciprocated in October that year and made a landmark visit to Damascus to seal ties.
But relations deteriorated from the onset of the Syria war and were finally severed, with Riyadh repeatedly calling for the end of Assad's regime.
Saudi officials have simultaneously chided the West for its reluctance to intervene militarily on the side of the armed opposition.
On Tuesday, the Saudi ambassador to Britain, Prince Mohammed bin Nawaf bin Abdul Aziz, published in The New York Times a bluntly worded assessment of the West's policies on Syria and Iran.
"We believe that many of the West's policies on both Iran and Syria risk the stability and security of the Middle East," he wrote in the commentary.
The senior diplomat said Saudi Arabia has "global responsibilities", both political and economic, and vowed it will continue to support the rebel Free Syrian Army and opposition fighters.
"We will act to fulfil these responsibilities, with or without the support of our Western partners," wrote the ambassador.
He also acknowledged the threat of Al-Qaeda-linked groups in Syria, arguing however that the best way to counter the rise of extremists among the rebels was to support the "champions of moderation".
Muqdad on Thursday told AFP that "Saudi Arabia should be put on the list of countries supporting terrorism."
Outside regime circles, there is also growing animosity towards Saudi Arabia.
Earlier this month, a film which depicts the Saudi royal family in an unflattering light was screened at the Damascus opera house.
"It was important for me to show this movie," said director Najdat Anzour of his "The King of Sands" movie, which opens with Al-Qaeda's 9/11 attacks on the US.
"Al-Qaeda didn't come from Mars but from Saudi Arabia, from the Wahhabi, extremist way of thinking," Anzour told AFP.
Anzour said a Saudi cleric has issued a fatwa, Islamic decree, authorising his killing.