WASHINGTON (AFP) - US lawmakers aimed to thread the needle on Tuesday (March 20 US time) on an enormous federal spending bill, rushing to meet a looming deadline before government funding expires, yet again, in three days.

If no action is taken by midnight on Friday (March 23), the US government could tumble into shutdown for the third time this year.

Republicans and Democrats have spent the past several weeks thrashing out a roughly US$1.2 trillion (S$1.58 trillion) deal on spending for the remainder of the fiscal year, which ends Sept 30.

House Speaker Paul Ryan said that "some unresolved issues" remained, but that he was hoping it would be finalised and released later in the day.

That would give lawmakers in the House of Representatives and Senate, both of which are controlled by Republicans, little time to study, debate and then pass the massive legislation before the deadline.

Still, congressional leaders remained hopeful that a shutdown could be avoided.

"A few sticking points remain, but we are very close" to a deal, top Senate Democrat Chuck Schumer said.

Schumer, President Donald Trump's chief rival in Congress, would not address some of the specifics involved in the final haggling.

"We're at the end of negotiations, I don't want to jeopardise anything," he said.

The measure follows the outlines of a 2018-2019 budget deal reached last month that boosts both military and domestic spending by more than 10 per cent.

Ryan was expecting "the biggest increase in defence spending in 15 years" - about US$80 billion above current spending limits - a move he said would reverse the damage caused by a decade of budget constraints on the armed forces.

Democrats can, meanwhile, claim increased domestic spending on issues like infrastructure, education and battling the opioid crisis.

But several politically sensitive riders - elements that are attached to must-pass legislation because they would have little chance of success on their own - were still up in the air, including an effort by Democrats to fund Obamacare subsidies granted to insurance companies serving low-income patients.

One outstanding issue is whether to boost border and immigration enforcement funding, including funding for Trump's pledged border wall, along with temporary protections for immigrants who arrived illegally in the country as minors.

Last-minute haggling was also taking place over language on abortion services funding and a measure that would tighten enforcement of background checks on gun purchases.

Also in play: possible federal funding for the multi-billion-dollar Gateway commuter rail tunnel between New York and New Jersey.

"This is a needed project and I hope Congress rises to the occasion," Schumer said.

But Trump is opposed to spending federal money on its construction, and it is unclear whether that would be enough to keep the provision out of the so-called "omnibus" spending bill.

With time ticking away, talk on Capitol Hill turned to the prospect of a possible short-term funding bill to avoid the embarrassing election-year prospect of a third government shutdown in as many months.

But Senate Majority Leader Mitch McConnell shot down the speculation. "My focus is on finishing this omnibus... and we're going to do it this week," he said.

Congress failed twice before to pass a 2018 federal spending bill before the deadline, sending the government into shutdowns that lasted a few days in January, and a few hours in February.

House conservatives were already expressing concern about the latest spending bill, projected to add billions to the US deficit just months after Trump signed a massive tax cut into law.

"Many of us Republicans have a major problem with the amount of spending that's going on here in Washington DC," House Republican Mark Walker told Fox News.

"We must show some true discipline" on fiscal issues, he added.