British Prime Minister David Cameron has said the resignation of the most senior Syrian diplomat serving in London is a serious blow to President Bashar al-Assad's regime in Syria which he hopes will "crumble and fall as fast as possible". Khaled al-Ayoubi, who had been charge d'affaires, quit his post in protest at the "violent and oppressive" actions of the regime of President Assad, the Foreign Office said on Monday. His departure was the latest in a series of diplomatic and military defections as the rebellion against Assad heads for its 18th month and the death toll approaches 19,000 by some estimates. Cameron said al-Ayoubi, an ethnic Kurd who joined the Syrian diplomatic service in 2001, was "basically defecting from the Syrian regime". "We want this regime to crumble and fold as fast as possible. Look what it's doing to its own people in Aleppo. Absolutely appalling behaviour. "So this is another step on the way to Syria having a proper transition."