The US Senate is voting on the first steps to end the 40-day government shutdown after a group of moderate Democrats agreed to proceed without a guaranteed extension of health care subsidies, angering many in their caucus who wanted to continue the fight.

The group of three former governors – Jeanne Shaheen and Maggie Hassan of New Hampshire and an independent, Angus King, of Maine – said they would vote to reopen if the Senate passed three annual spending bills and extend the rest of government funding until late January.

Senate Majority Leader John Thune endorsed the deal Sunday night and called an immediate vote to begin the process of approving it.

The deal would also include a future vote on the health care subsidies, which would not have a guaranteed outcome, and a reversal of the mass firings of federal workers that have happened since the shutdown began on October 1. The full text of the deal has not yet been released.

Republicans need five Democratic votes to reopen the government. In addition to Shaheen, King and Hassan, Democratic senator Tim Kaine of Virginia, home to millions of federal workers, also said he would support the agreement.

After Democrats met for over two hours to discuss the proposal, Senate Democratic leader Chuck Schumer said he could not "in good faith" support it.

"America is in the midst of a Republican-made health care crisis," Schumer said on the floor just ahead of the expected votes, adding that Americans would "suffer immensely" and that the crisis would only get worse.

"Democrats have sounded the alarm," Schumer said, and "will not give up the fight".

Final passage of the legislation could take several days if Democrats object and draw out the process. Independent senator Bernie Sanders, who caucuses with the Democrats, said that giving up the fight was a "horrific mistake".

Republicans have been working with the group of moderates as the shutdown continued to disrupt flights nationwide, threaten food assistance for millions of Americans and leave federal workers without pay.

But many Democrats have warned their colleagues against giving in, arguing that they can't end the fight without an agreement to extend the health subsidies.

Democrats have now voted 14 times not to reopen the government as they have demanded the extension of tax credits that make coverage more affordable for health plans offered under the Affordable Care Act.

Republicans have refused to negotiate on the health care subsidies while the government is closed, but they have been supportive of the proposal from moderate Democrats as it emerged over the last several days.

The agreement would fund parts of government – food aid, veterans programmes and the legislative branch, among other things – and extend funding for everything else until the end of January.

It would take up Republicans on their longstanding offer to hold a future vote on the health care subsidies, with that vote occurring by the middle of December, the people said. (AP)