US Democrats reached a compromise late on Friday on key elements of President Joe Biden's massive COVID-19 relief package, breaking a logjam and setting the US$1.9 trillion stimulus bill on a path toward a final vote.

Negotiations in the evenly divided Senate had frozen the chamber's action for some nine hours, with concern mounting that Democratic unity was fraying and imperiling the huge measure, before the breakthrough.

Votes on more amendments to the bill were set to occur through the night on Friday and into Saturday, with a final vote expected some time this weekend.

The breakthrough came against a backdrop of strong US economic data signalling that the world's largest economy may finally be healing.

They included better-than-expected hiring in February as businesses battered by the pandemic began recruiting employees again.

Payrolls jumped by 379,000 last month, which was almost double expectations and pushed the unemployment rate down slightly to 6.2 percent, the Labor Department reported.

The vast majority of the gains were in the leisure and hospitality sector, which was devastated in the pandemic's early months.

Yet the economy was still short 9.5 million jobs compared to February 2020, before the pandemic began, the report said.

Biden said the data underscored the need for lawmakers to approve his plan for nearly US$1.9 trillion in aid, and his economic advisers said the current pace of job gains mean it would take two years to recover to pre-pandemic levels.

"We can't go one step forward and two steps backward," Biden said at the White House. "The rescue plan is absolutely essential to turning this around."

But on Friday the whole bill appeared to be under threat.

The Senate impasse was over the amount and duration of supplemental unemployment insurance benefits for people left jobless during the pandemic.

Democrats had struggled to keep moderate Senator Joe Manchin's support for expanded unemployment aid in the measure, and he threatened to side with Republicans who offered a counterproposal that they say would save US$128 billion.

Keen on more fiscal restraint than was evident in the Biden package, Manchin balked at an amendment that would increase weekly additional unemployment benefits from US$300 to US$400 and extend them through August.

But ultimately he agreed to a compromise that reduced the jobless benefits back to US$300 but extended them to September 6.

It also provides for tax breaks of up to US$10,200 for those receiving unemployment, but only for families making less than US$150,000.

"We have reached a compromise that enables the economy to rebound quickly while also protecting those receiving unemployment benefits" from being hit with unexpected tax bills, Manchin said in a statement.

'Vote-a-rama'

Biden supports the deal, White House press secretary Jen Psaki said.

"Most importantly, this agreement allows us to move forward on the urgently needed American Rescue Plan, with US$1,400 relief checks, funding we need to finish the vaccine rollout, open our schools, help those suffering from the pandemic, and more," she said in a statement.

An effort to include a national minimum wage hike to US$15 an hour — a Biden priority — failed, however.

Biden's proposal would be the third major stimulus package to help the economy weather the coronavirus crisis.