Syria's ambassador to Iraq has defected to the opposition and urged other senior Syrian politicians and members of the military to do the same. Nawaf Fares is the first senior Syrian diplomat to abandon the government of President Bashar al-Assad. The move comes just a week after a Syrian general from a powerful family close to President Assad also defected. Meanwhile Western nations are urging the UN to threaten tough sanctions against Syria. The development comes as the Security Council opens talks on the future of its observer mission in the country. It must pass a new resolution before the mission's mandate ends on Friday next week. Mr Fares confirmed his defection in a statement broadcast on Facebook and al-Jazeera TV. With Syrian revolutionary flags behind him, he read out the statement saying he was resigning both as Syria's ambassador to Iraq and as a member of the ruling Baath Party. "I call on all party members to do the same because the regime has transformed it into a tool to oppress the people and their aspirations to freedom and dignity. "I announce, from this moment on, that I am siding with the people's revolution in Syria, my natural place in these difficult circumstances which Syria is going through." Tribal chief Mr Fares was appointed ambassador to Baghdad in 2008. The BBC's Jim Muir in neighbouring Lebanon says his is a highly damaging defection for President Assad. Mr Fares, significantly, is also chief of a Sunni tribe, the Uqaydat, which straddles Syria's eastern border with Iraq, our correspondent adds. That area, around the city of Deir al-Zour, has become a hotbed of support for the rebels and has been heavily bombarded in recent weeks. Syria has been convulsed by internal conflict since protests against President Assad began early last year. The protests turned into an armed rebellion and thousands of people have been killed. Last week, senior army officer Brig Gen Manaf Tlas fled Syria via Turkey. The BBC's Barbara Plett at the UN says Moscow has already rejected such measures as "unhelpful", suggesting tense negotiations in the coming days. In a closed-door briefing, Mr Annan called on the council to send a message to the Syrian government and the rebels that there would be "clear consequences" for failing to abide by the ceasefire. Our correspondent says Western diplomats seized on this to bolster their case for sanctions. Military action The UK's envoy to the UN, Mark Lyall Grant, told reporters that Britain, France, the US and Germany would propose making compliance with the ceasefire mandatory. They would invoke Chapter Seven of the UN charter, which would allow the use of military action to enforce the resolution. US envoy Susan Rice said the mandate could not simply roll over. "Our view is that this council needs to put that kind of plan under Chapter Seven, make it clear that it is binding," she said. Russian deputy envoy Alexander Pankin played down the possibility of military action. "Chapter Seven is a last resort. Chapter Seven is not a very efficient mechanism," he said. China, which also vetoed the two previous attempts to impose tougher measures, said it would support a rollover of the mission. He was a commander of a unit of the elite Republican Guard and as a young man he attended military training with President Assad. Gen Tlas had been under a form of home arrest since May 2011 because he opposed security measures imposed by the regime, sources said. Meanwhile diplomatic efforts to resolve the Syrian crisis are continuing, with Western nations pressing the UN to threaten Damascus with sanctions. The UN is currently debating the mandate for its observer mission in Syria which expires on Friday. The mission had a 90-day remit to monitor a truce, but fighting has continued largely unabated. The truce formed part of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan. Russia has suggested a 90-day extension but Western states say it is not enough and they want to include the threat of sanctions if the Syrian government continues its military campaign against the opposition.