via Bangkok Post, 18 September 2018: Thailand intends to nominate Si Thep as a Unesco World Heritage Site.

Thailand is preparing to propose Sri Thep Historical Park in Phetchabun as a World Heritage Site, following on the footsteps of the 700-year-old city of Sukhothai and Ban Chiang archaeological site in Udon Thani.
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Sri Thep Historical Park has been listed as a national archeological site since 1935.
The ancient city was once an important cultural centre in the region, spanning across parts of the lower north provinces of Thailand, including Phetchabun, Phitsanulok, Tak, Sukhothai and Uttaradit.
Source: Sri Thep proposed as World Heritage Site | Bangkok Post: news