The US Senate has taken a first concrete step towards dismantling the Affordable Care Act, popularly known as Obamacare, voting to instruct key committees to draft legislation repealing President Barack Obama's signature health insurance scheme.

Following the 51-48 vote on Thursday, the resolution now will go to the House of Representatives, which is expected to vote on it this week.

Scrapping Obamacare is a top priority for the Republican majorities in both chambers of the US Congress and for President-elect Donald Trump.

Republicans have said the process of repealing Obamacare could take months, and developing a replacement plan could take longer.

However, they are under pressure to act fast from Trump, who said on Wednesday that the repeal and replacement should happen "essentially simultaneously".

Some 20 million previously uninsured Americans gained health coverage through Obamacare. Coverage was extended by expanding Medicaid and through online exchanges where consumers can receive income-based subsidies.

Republicans have launched repeated legal and legislative efforts to unravel the law, criticising it as government overreach. They say they want to replace it by giving states, not the federal government, more control.

In recent days, though, some Republicans expressed concern about the party's current strategy of voting for a repeal without having a consensus replacement plan ready.

January 27 target date

House Speaker Paul Ryan said this week he wants to pack as many replacement provisions as possible into the legislation repealing Obamacare. But Senate finance committee chairman Orrin Hatch, also a Republican, said this could be difficult under US Senate rules.

The resolution approved on Thursday instructs committees of the House and Senate to draft repeal legislation by a target date of January 27. Both chambers will then need to approve the resulting legislation before any repeal goes into effect.

Republicans have a majority of 52 votes in the 100-seat Senate; one Republican, Senator Rand Paul, voted no on Thursday.

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Democrats mocked the Republican effort, saying Republicans had never united around an alternative to Obamacare.

"They want to kill ACA but they have no idea how they are going to bring forth a substitute proposal," declared Senator Bernie Sanders of Vermont.

Trump said on Wednesday he would submit a replacement plan as soon as his nominee to lead the Health and Human Services department, Tom Price, is approved by the Senate. He gave no details.

Democrats passed the Affordable Care Act in 2010 despite united Republican opposition.