It's reported that peace negotiations have already begun in secret.

Literal

Peace-negotiations [subject-が] already secretly [manner-に] were-started [hearsay-とのことです].

~とのことです is a formal hearsay/report marker: 'it is said that, according to reports.' 秘密裡に (ひみつりに, 'in secret') uses the literary suffix 裡 ('within'). 開始された is passive, giving an impersonal news-report tone.