US Senator Kyrsten Sinema has announced her intention to leave the Democratic Party, four years after being elected.

But she emphasised that she will not sit with Republicans and said she would instead serve as an independent member in the chamber.

The Democrats maintain control of the upper chamber with Vice-President Kamala Harris's tie-breaking vote.

The Arizona lawmaker said she made the move to fight what she called a "broken partisan system".

"Registering as an independent and showing up to work with the title independent is a reflection of who I've always been, and it's a reflection of who Arizona is," she said in a Twitter video.

"We don't line up to do what we're told, we do what's right for our state and our country."

In an opinion piece written for the Arizona Republic newspaper, she said that "Americans are increasingly left behind by national parties' rigid partisanship" and that she hoped to "work proudly with senators in both parties".

She joins Bernie Sanders of Vermont and Angus King of Maine - who both sit with the Democratic party - as the chamber's only independent senators.

The 46-year-old did not address whether she will run for re-election in 2024, and informed Democratic Leader Chuck Schumer of her decision on Thursday. US media also reported that she had informed the White House of her intention.

Practically, the decision might make little difference to the Senate after Democratic Senator Rafael Warnock's win in Georgia earlier this week gave the party a 51-49 majority in the chamber.

Senator Sinema has long been willing to vote against the party line on a variety of issues, and some of her actions have angered local party officials in Arizona.

She was the last Democrat to hold out against President Joe Biden's $700bn (£577bn) climate and tax bill in August.

And her refusal to abandon the filibuster - a mechanism rule which requires 60% majority to pass certain legislation in the Senate - infuriated some colleagues earlier this year.

Even before her switch she had faced rumours of a primary challenge in 2024 from Congressman Ruben Gallego.

In recent years the Republican party has lost its once vice-like grip over Arizona politics. Senator Sinema's victory over Martha McSally in 2018 was the first of a wave of Democratic Party victories.

Since then, former astronaut Mark Kelly has taken the state's other senate seat, while Katie Hobbs was elected governor in last month's mid-term elections.

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