iCloud Backup, Photos, iCloud Drive, iCloud Keychain and more suffering outages

Apple has updated its Systems Status webpage to show several iCloud services including iCloud Backup, Photos, iCloud Drive, iCloud Keychain and iCloud Bookmarks & Tabs as suffering outages affecting some users, causing the services to be slow or unavailable.
First spotted by 9to5Mac, Apple states online that the outages started around 2:54 a.m. PT, with other iCloud services such as iCloud Mail, iCloud Reminders and iCloud Web Apps on iCloud.com appearing unaffected by the ongoing disruption.
The reason behind the outages is unknown, with Apple simply noting that the downtime is “ongoing” which means engineers are actively working on the issue.