The US agency responsible for maintaining and designing the nation’s cache of nuclear weapons was among those breached by a hack of Microsoft Corp.’s SharePoint document management software, according to a person with knowledge of the matter.

The tech giants have evidence that Chinese hackers are exploiting the new bug, but warned "multiple actors" are also hacking into affected SharePoint systems.

Microsoft said it identified 2 groups as leveraging flaws in SharePoint software; China said it firmly opposes all forms of cyberattacks.