Nawaf Fares, who this week defected as Syria's ambassador to Iraq, on Saturday accused President Bashar al-Assad of allowing Al-Qaeda to use Syria as a springboard for attacks in his former host country.
Fares, the latest high-level official to abandon Assad, in an interview on Al-Jazeera television also accused Iraqi Prime Minister Nuri al-Maliki of taking a stance toward Syria that was "contradictory" to the truth.
Majority Shiite Muslim Iraq had repeatedly accused Damascus of letting Sunni insurgents and arms transit through Syria to carry out attacks inside the country, especially during the brutal sectarian conflict that erupted after US-led forces toppled Saddam Hussein's regime in 2003.
But since the uprising broke out last year against Assad -- who belongs to the Alawite offshoot of Shiite Islam -- Iraq's Shiite-led government has called for non-interference in Syria and has opposed arming Syrian rebels.
"I personally reproach the (Iraqi) prime minister on his stance, which is contradictory to the truth," Fares said.
"He knows very well what Bashar al-Assad had done to him and to all of Iraq and to Shiites specifically," Fares said, adding that Assad has "killed thousands" by opening "the doors for Al-Qaeda" militants to carry out bombings across Iraq.
Baghdad's stance was similar to that of Syria's other Shiite ally, Iran, which Fares accused of "putting pressure" on Maliki's government.
"Iran must not support a tyrant and dictator who is killing his own people, regardless of its interests," Fares told Al-Jazeera in Qatar, to which he travelled after quitting his post.
Western countries and the Syrian opposition accuse Iran of providing military support to the regime in Syria, where activists say more than 17,000 people have been killed since March 2011. Tehran denies the charge.
When asked about UN-Arab League envoy Kofi Annan's bid to get Iran's support for his tattered peace plan, Fares said: "Iran is part of the problem. How could it be part of the solution?"
"The Syrian revolt will win despite Iran and all countries backing the tyrant," whom Fares described as "Syria's former president who is now a criminal and a killer."
Fares, the first Syrian ambassador to defect to the opposition, was widely seen as a regime hardliner and his decision, announced on Al-Jazeera on Wednesday, was surprising.