Syria's ambassador to Iraq Nawaf Fares has defected to the opposition and urged the army to turn its guns on the regime, becoming the most senior diplomat to abandon President Bashar al-Assad.

"I announce my defection from my post as representative of the Arab Syrian Republic in Iraq and my withdrawal from the ranks of the (ruling) Baath party," Fares said in a message aired on Al Jazeera TV Wednesday and confirming earlier reports of his defection.

"I call on all free and worthy people in Syria, particularly in the military, to immediately rejoin the ranks of the revolution," he said, adding: "turn your cannons and your tanks towards the criminals in the regime who are killing the people.

He also called on fellow Baath party members to reject the regime which he accused of using the party as "a tool to repress the people and their aspiration of freedom and dignity."

The disaffected diplomat submitted a letter to the Iraqi foreign ministry and will meet with Iraqi officials on Thursday, a diplomat said.

"They are going to discuss sending him to another country," the diplomat added.

In Washington, White House spokesman Jay Carney told reporters: "There have been a number of high-level defections in recent days and weeks, and they are simply the tip of the iceberg."

"There have been many, many defections within the military leadership, within the government, and I think that is an indication of the fact that support for Assad is crumbling, internationally and internally. And that's a welcome development."

General Munaf Tlass is thus far the most influential military officer to have abandoned the Assad regime.

An officer in the elite Republican Guard charged with protecting the regime, he is the son of former defense minister Mustafa Tlass, a close friend of Assad's late father and predecessor, Hafez.

Pressure is mounting on Assad as Western powers drew up a 10-day sanctions ultimatum late Wednesday.

At the United Nations, Britain, France, Germany and the United States submitted a draft text that would give Assad 10 days to implement UN-Arab League envoy Kofi Annan's ceasefire plan or face tough new sanctions.