Fēichángguī wǔqì shòudào guójì tiáoyuē de xiànzhì.
Unconventional weapons are restricted by international treaties.
Tips
grammar
非常规 functions as an attributive adjective before nouns: 非常规 + 方法 / 武器 / 手段. Note that 非常规 (fēi cháng guī) is different from 非常 (fēicháng — 'very/extremely'): the 规 (rules/regulations) is what makes 非常规 mean 'outside the norm'.