During this period, the Tibetans strengthened and expanded their empire. By the late 660s they had overrun the Tuyuhun, and were in direct contact with Tang territory. The two empires fought sporadically over the following decades, and much of North western China fell to the Tibetan hands. With the weakening of Chinese power consequent on the An Lushan Rebellion (755–763), the Tibetans managed to recapture vast swathes of their lost territory, overrunning Songzhou and the surrounding area in 763, and even briefly capturing the capital Chang'an. Songzhou (Tibetan "Sharkok" ) was thereafter reportedly settled by Tibetans from Ngari and identified in Tibetan geographical writings as part of the expanding Tibetan empire, classified variously as part of Amdo or Kham. Sharkok and neighboring Khöpokok (Jiuzhaigou) remain Tibetan-speaking areas to this day (previously classified as Amdo Tibetan, now tentatively classified as five distinct dialects of an independent branch of Tibetan, "Sharkhog Tibetan".).