noun
diplomatic source; diplomatic circles
Used in news reports to refer to an unnamed source within the diplomatic community, similar to 'diplomatic sources say...'.
外交筋によると、首脳会談は来月開かれる見通しだ。
According to diplomatic sources, the summit meeting is expected to be held next month.
消息筋 is a broader term for 'informed sources' in general, while 外交筋 specifically refers to diplomatic sources.
Compound of 外交 (diplomacy) and 筋 (channel, source).