The US government was on the verge of a shutdown on Friday as Congress struggled to reach a deal to keep agencies running, prevent national park closures, and avoid the suspension of salaries for federal employees and military staff.

The closure, set to start after midnight on Saturday (0400 GMT Sunday), would be the first since 2019 and appears increasingly inevitable, with lawmakers at an impasse on approving even a short-term spending bill.

The two chambers of Congress are deadlocked, with a small group of hardline Republicans in the House of Representatives pushing back against any stop-gap measure that would at least keep the lights on.

On Friday, House Republicans unveiled a plan to extend funding until the start of November, but only if there were deep spending cuts of about 30 percent across the board, with exemptions in areas such as defence and disaster relief.

President Joe Biden's administration said in a statement that that he would veto this.

"They are breaking their word, abandoning the bipartisan deal that two-thirds of them voted for just four months ago, and marching our country toward an Extreme Republican Shutdown that will damage our economy and national security," White House Press Secretary Karine Jean-Pierre said in another statement.

Republican House Speaker Kevin McCarthy, however, blames the Democrats, saying they are the ones blocking a solution.

A shutdown would mean the majority of national parks – from the iconic Yosemite and Yellowstone in the west to Florida's Everglades swamp – would be closed to public access beginning on Sunday.

Only areas that are physically accessible to the public will remain open with reduced services, according to the Department of the Interior.

These include the National Mall in Washington and Gateway National Park in San Francisco, alongside lookouts, campgrounds and trails.

"At National Park Service units across the country gates will be locked, visitor centres will be closed and thousands of park rangers will be furloughed," said a senior official.

The official said that in the past, state authorities have sometimes stepped in to fill the federal gap in parks funding.

A shutdown places the world's largest economy at risk "unnecessarily at a time when it is doing so well," said White House National Economic Council director Lael Brainard on CNBC Friday.

Among potential risks are air travel delays, with air traffic controllers asked to work without pay, while households may be unable to access certain benefits, she added.

With student loan repayments set to resume in October, officials also told reporters on Friday that key activities at Federal Student Aid will continue for a couple of weeks if a shutdown happened.

"However, a prolonged shutdown lasting more than a few weeks could substantially disrupt the return to repayment effort and long term servicing support for borrowers," said Zayn Siddique of the White House Domestic Policy Council.

A State Department spokesperson added that it is crucial for Congress to reach a deal and support a request by the Biden administration for supplemental funding for Ukraine and other matters.

These include global humanitarian needs and funds to help with the migration crisis. (AFP)