Al-Fares urged others in Syria to join him in abandoning the regime of President Bashar Assad, the Arab-language broadcaster al-Jazeera reported, saying it had obtained al-Fares' statement exclusively.
"I announced my resignation as Syrian ambassador to Iraq as I also declare my defection from the Syrian Baath party," al-Jazeera quoted Fares as saying. "I urge all honest members of this party to follow my path because the regime has turned it [the party] to an instrument to kill people and their aspiration to freedom."
The ambassador urged the Syrian military to join the rebels.
The reaction from the Obama administration in Washington was reserved. State Department spokesman Patrick Ventrell said Washington officials had seen the news reports "but cannot confirm them."
"If true, this would be the first senior diplomat from the regime to defect and is just further proof that the regime is getting weaker and losing its grip on power," Ventrell told reporters. "There have been countless defections from the Syrian military and security forces who have courageously rejected the horrific actions of the Assad regime. So if true, we welcome it."
White House spokesman Jay Carney likewise said he was unable to confirm the diplomat's defection.
"I can tell you that there have been a number of high-level defections in recent days and weeks, and they are simply the tip of the iceberg," Carney said. "There have been many, many defections within the military leadership, within the government, and I think that that is an indication of the fact that support for Assad is crumbling -- internationally and internally. And that's a welcome development.
"And we have, as you know, called on those around Assad to consider their options here, to see what Assad is doing to his own people, to understand that siding with Assad is allying with a tyrant who will go down in history as such, and that the right choice is to abandon him, to abandon the regime, and support the Syrian people in their pursuit of a better future."
The head of the Syrian National Council said he would discuss proposals to resolve the Syrian crisis Wednesday with Russian Foreign Minister Sergey Lavrov.
Abdul Basit Sieda, who heads the opposition group, said he would bring up the proposals during a meeting in Moscow with Lavrov, Russia's official ITAR-Tass news agency reported.
Russia has opposed international demands Assad be forced from power, which opposition groups have sought, and Russia and China have vetoed draft U.N. Security Council resolutions that would have formally condemned the Syrian regime.
Soon after taking over as head of the SNC last month, CNN said, Sieda had urged officials in Syria, Russia and China "to think carefully about the situation now because the whole stability of the region, if not the whole stability of the world, is at stake here. We would like to call upon them to support the Syrian people."
At least 34 people were killed in Syria Wednesday, the opposition Local Coordinating Committees of Syria said. Opposition groups say as many as 17,000 people have died since the uprising against Assad began 16 months ago.
U.N.-Arab League special envoy Kofi Annan was to brief the U.N. Security Council Wednesday after diplomatic attempts to end the violence.
Annan visited Iran and Iraq Tuesday and said he views Iran as a factor in diplomatic efforts to bring peace to Syria.
Russia has tabled a draft resolution suggesting a three-month extension of the U.N. observer mission in Syria, which has been suspended because of violence, and the Security Council must decide what to do with the 300 U.N. observers.
Annan met Monday with Assad in Damascus and said Assad "made a suggestion of building an approach from the ground up in some of the districts where we have extreme violence -- to try and contain the violence in those districts and, step by step, build up and end the violence across the country."
Annan said he did not want to mention details about efforts to end the violence until he talked with opposition leaders.