Donald Trump says Venezuela's embattled opposition leader must be protected, as President Nicolas Maduro prepares to take the oath again after allegedly stealing an election.

Nicolás Maduro took the oath before parliament, vowing his third six-year term in office would be a "period of peace".

The United States on Friday imposed new sanctions on eight Venezuelan officials and increased the reward for the arrest of President Nicolas Maduro on the day of his inauguration to a third term following a disputed election last year.