Former US congressman Anthony Weiner has begun his 21-month prison sentence for illicit online contact with a 15-year-old girl.

Prosecutors said he broke the law by having illicit contact with a 15-year-old girl using Skype and Snapchat.

The FBI told a federal court it needed a search warrant to look at thousands of Hillary Clinton's emails on the laptop of former US congressman Anthony Weiner because they had potential to cause "grave damage to national security" if disclosed.