A hacking group that appears to be linked to the Iranian government attempted to break into U.S. President Donald Trump's re-election campaign but were unsuccessful, sources familiar with the operation told Reuters on Friday.

Microsoft said earlier on Friday that it saw "significant" cyberactivity by the group which also targeted current and former U.S. government officials, journalists covering global politics and prominent Iranians living outside Iran, the company said in a blog post.

Republican Trump's official campaign website is the only one of the remaining major contenders' sites that is linked to Microsoft's cloud email service, according to an inspection of publicly available mail exchanger records.