Image copyright Getty Images Image caption The legislation affects tens of millions of Americans and about a sixth of the US economy

Four Republican senators have expressed reservations about their party's plan to repeal Obamacare, throwing the health bill's fate into uncertainty.

Ted Cruz, Ron Johnson, Mike Lee and Rand Paul said they were "not ready to vote for this bill", but were "open to negotiation".

The Senate's Republican leader unveiled his plan to overhaul the US healthcare system after drafting it in secret.

The party can only afford to have two defections to pass the measure.

Republicans need to secure 50 votes when the bill comes to the floor next week. No Democrats are expected to support it.

Forty-three people were arrested on Thursday while protesting against the legislation outside the office of Senate Majority Leader Mitch McConnell, who crafted most of the plan.

The statement from the four senators, who are from the more conservative wing of the party, said: "Currently, for a variety of reasons, we are not ready to vote for this bill, but we are open to negotiation and obtaining more information before it is brought to the floor."

Media playback is unsupported on your device Media caption Trump's battles with Obamacare - in his own words

The four conservatives are concerned the legislation is not hardline enough, however, two more moderate Republican senators voiced fears it might be too severe.

The bill would slash taxes on the wealthy as well as insurance and drug companies, all paid for by steep cuts to Medicaid, a government medical programme for the poor and the majority of older Americans in nursing homes.

Senator Dean Heller - who faces re-election next year - said he had "serious concerns" about the bill's impact on his home state of Nevada.

"As I have consistently stated, if the bill is good for Nevada, I'll vote for it and if it's not - I won't," he said in a statement.

Susan Collins of Maine said it was "too soon" to decide on whether she would vote for the bill.

She expressed "concerns" about the proposed Medicaid cuts and eliminating funding for the women's health group Planned Parenthood for a year.

Media playback is unsupported on your device Media caption Healthcare battle in rural America

The 142-page "discussion draft" scraps most of President Barack Obama's 2010 signature health law, including dropping the requirement for individuals to have health insurance.

Repealing Mr Obama's law, called the Affordable Care Act but also known as Obamacare, was a central promise of Republicans throughout his presidency.

In a Facebook post, Mr Obama said there was a "fundamental meanness" at the core of the Republican bill.

He described it as "a massive transfer of wealth from middle-class and poor families to the richest people in America".

The Senate's legislation follows the House of Representatives version, which passed the lower chamber six weeks ago.

The proposals affects tens of millions of Americans and a sixth of the US economy.

What's in the Senate bill?

"Republicans believe we have a responsibility to act - and we are," Mr McConnell said after presenting the Better Care Reconciliation Act Of 2017 to the upper chamber on Thursday.

The bill mostly aligns with the House plan, but ties federal subsidies for individuals based on their income rather than age - as the Affordable Care Act currently does.

It would make it more difficult for recipients to qualify for those subsidies, however, by implementing more restrictions on income requirements.

Critics of the House bill, which tied the subsidies to age, say it unfairly penalised older Americans.

The Senate plan phases out the expansion of Medicaid more gradually than the House bill.

But it would impose deeper long-term cuts to the programme.

The bill also gives states more latitude in requiring insurers to provide essential health benefits guaranteed under Obamacare, including emergency and maternity care and mental health services.

The great unveiling - Anthony Zurcher, BBC News North America reporter

The curtain has finally come up on the Senate healthcare bill, and the product of weeks of back-room negotiations looks a lot like the oft-derided House version.

Sure, there are a few key differences. In some places, it's more moderate - keeping in place income-adjusted subsidies to help less affluent Americans purchase insurance, for instance.

In other areas the cuts are actually much more aggressive.

It appears Senate Majority Leader Mitch McConnell hopes to mollify moderates and hard-liners by pushing the painful changes to low-income insurance programmes farther down the road.

Whether that's enough to cobble together the bare-minimum 50 votes necessary from the 52 Republicans in the Senate will be the big question next week.

They party has spent the last seven years saying it would tear Obamacare up "root and branch", in Mr McConnell's words, and they can't risk walking away empty handed.

What happens next?

Senator McConnell expects the bill to come to the Senate floor as early as next week, when the non-partisan Congressional Budget Office releases the plan's estimated cost and impact on Americans.

The Republican Senate leadership is hoping to pass the measure, before sending it back to the House for approval of the new draft.

The House would need to pass the bill as it stands and send it to President Donald Trump's desk to sign into law or draft another version, which both chambers would again have to approve.

Mr Trump has previously referred to the House version as "mean" and implored senators to draft a more "generous" version.