Apparently the border between the -stan countries and China often see trespassers entering into China (yes it's fenced up). Some of them are herders crossing boundaries somehow. Others are Islamist militants entering China.

显然，在中亚的“斯坦”国家和中国的边境总是有非法入境者进入中国（是的，有栅栏）。他们中有些是牧民，不知怎的跨越了边境。其他进入中国的是伊斯兰激进分子。

The Men and Women Who Fight China's 'Anti-Terrorist' War <- Read this.

为中国“反恐”战而斗争的战士们<-读读这篇文章吧。

From another Foreign Policy report Chinese security forces see an increase in militants despite arresting many. Apparently they stream in through broken fences. Every now and then they will cause trouble China uses flamethrower to hunt Xinjiang 'terrorists' | The Japan Times