Hillary Clinton will end months of speculation and launch her highly anticipated US 2016 presidential campaign on Sunday, attempting a second bid at becoming the US' first female president, American media outlets are reporting.

Clinton, the former first lady and secretary of state who lost the 2008 nomination to Barack Obama, is expected to make her announcement via a video posted to social media, news agency The Associated Press reported, quoting two people familiar with her campaign.

Clinton appears unlikely to face a formidable primary opponent, though a handful of lower-profile Democrats have said they are considering running for president.

Some liberals have tried to lure Senator Elizabeth Warren into the race, but she has rejected the idea.

Should she win the nomination, Clinton would face the winner of a Republican primary field that could feature as many as two dozen candidates.

They could include Jeb Bush, the Florida governor, and brother and son of former presidents, and Senator Marco Rubio of Florida, who is expected to announce his campaign in Miami on Monday.

The announcement will mark Clinton's formal return to politics following a two-year leave from government.

In the past few weeks, Clinton has been criticised over her use of a personal email account and server while she was secretary of state, as well as over the Clinton Foundation's acceptance of donations worth millions of dollars from foreign governments during her tenure in the State Department.

Republicans reviewing the 2012 attack on the US embassy in Benghazi, Libya, which took place during Clinton's watch, are also investigating her decision to delete thousands of emails she has deemed personal in nature.

Source: Agencies