Rebel fighters killed at least 18 soldiers as they overran a military post near Syria's northeastern border with Turkey on Wednesday, the Syrian Observatory for Human Rights said.
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The clashes -- which also left an unknown number of soldiers wounded -- took place near Ras al-Ain, a border town that the opposition fighters seized on Friday, said the Britain-based watchdog.
It also said that some soldiers managed to flee the area, located in the province of Hasakeh.
Three rebels were killed and more than 10 wounded, said the watchdog, which relies on a network of activists, lawyers and medics in civilian and military hospitals.