US President-elect Donald Trump is facing a new challenge from Green Party candidate Jill Stein. The minor party nominee has launched a recount effort in three swing states, saying she wants to guarantee the integrity of the US voting system. And Wisconsin's election board has approved her request.

The US election was nearly three weeks ago but now a minor party has launched a recount effort in three key swing states: Michigan, Wisconsin and Pennsylvania. Jill Stein was the Green Party nominee for President and only received about one percent of the vote nationwide.

"We are standing up for a voting system that we deserve, that we can have confidence in, that has integrity and security," said Jill Stein.

The Green Party says the effort is meant to account for irregularities from potential hacking and other vote tampering, but they have presented no evidence of wrongdoing.

"I want to be clear, there is no smoking gun. There is not something that we're pointing to say that this is wrong in Wisconsin, Michigan and Pennsylvania. What we are saying is that there's enough peripheral evidence to warrant that our system should be investigated," said George Martin, Green Party member.

Votes are still being tallied in many states and Hillary Clinton leads the popular vote count by about 2.2 million votes, but the US election is determined by electoral votes allocated by winning individual states. Donald Trump leads the Electoral College count. 

Vote tallies in Michigan, Wisconsin and Pennsylvania are close and if all three states shifted to Clinton, she would win the election. The Clinton campaign has now joined the recount effort, but there is very little chance it will change the election results.

Donald Trump responded with a series of tweets bashing the recount effort while his campaign accused Clinton of being a sore loser.

"She congratulated him and she conceded to him on election night, I was right there. And the idea that we're going to drag this out now where the president-elect has been incredibly magnanimous to the Clintons' and the Obamas' is pretty incredible," said Kellyanne Conway, Trump senior adviser.

The Electoral College meets December 19th to formally select the next U.S. president.