THE US federal government shutdown only partially curbs operations. But the longer the shutdown continues, the more likely its impact will be felt.

US troops will stay at their posts and post will get delivered, but almost half of the two million civilian federal workers will be barred from doing their jobs if the shutdown extends into Monday.

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Many State Department operations will continue, such as passport and visa processing, which are largely self-funded by consumer fees. The agency's main headquarters in Washington, in consultation with the nearly 300 embassies, consulates and other diplomatic missions around the world, will draw up lists of non-essential employees who will be put on leave.

State Department operations will continue through the weekend and staff will be instructed to report for work as usual on Monday to find out whether they have been put on leave.

The US military will continue to fight wars and conduct missions around the world, including in Iraq, Syria and Afghanistan. And members of the military will report to work, though they will not get paid until Congress approves funding.

However, Defence Secretary Jim Mattis warned on Friday that a shutdown will have far-reaching effects.

The workforce at the 17 US intelligence agencies will be pared down significantly, according to a person familiar with contingency procedures.

The official, who was not authorised to publicly discuss the matter and spoke on condition of anonymity, said employees who are considered essential and have to work will do so with no expectation of a regular paycheck.

While they can be kept on the job, federal workers cannot be paid for days worked during a shutdown. In the past, however, they have been paid retrospectively even if they were ordered to stay home.

The Interior Department says national parks and other public lands will remain as accessible as possible. That position is a change from previous shutdowns, when most parks were closed and became high-profile symbols of dysfunction.

Spokeswoman Heather Swift said the American public – especially veterans who come to the nation's capital – should find war memorials and open-air parks available to visitors. Swift said many national parks and wildlife refuges nationwide will also be open with limited access when possible.

She said public roads that already are open are likely to remain open, though services that require staffing and maintenance such as campgrounds, full-service bathrooms and concession stands will not be operating. Backcountry lands and culturally sensitive sites are likely to be restricted or closed, she said.

A department spokesman said nearly 90 per cent of Homeland Security employees are considered essential and will continue to perform their duties during a government shutdown.

That means most Customs and Border Protection and Transportation Security Administration workers will stay on the job, according to the department's shutdown plan.

Immigration and Customs Enforcement will be staffed at about 78 per cent, meaning more than 15,000 of the agency's employees will keep working. The Secret Service, also part of Homeland Security, will retain more than 5700 employees during the shutdown.

A shutdown plan posted on the Treasury Department's website shows that nearly 44 per cent of the Internal Revenue Service's (IRS) 80,565 employees will be exempt from being put on leave during a shutdown.

That would mean nearly 45,500 IRS employees will be sent home just as the tax agency is preparing for the start of the return filing season and ingesting the sweeping changes made by the new Republican tax law.

Half of the more than 80,000 employees of the Health and Human Services Department (HHS) will be sent home. Key programs will continue to function because their funding has ongoing authorisation and does not depend on annual approval by Congress. But critical disruptions could occur across the vast jurisdiction of HHS programs – including the seasonal flu program.

Deep into a tough flu season, the Centres for Disease Control and Prevention (CDC) will be unable to support the government's annual seasonal flu program. And CDC's ability to respond to disease outbreaks will be significantly reduced.

Many of the nearly 115,000 Justice Department employees have national security and public safety responsibilities that allow them to keep working during a shutdown. Special counsel Robert Mueller's team investigating Russian meddling in the presidential election will also continue working. His office is paid for indefinitely.

Environmental Protection Agency (EPA) administrator Scott Pruitt has instructed workers there to come to work next week even with a shutdown. Pruitt said in an email to all EPA employees on Friday that the agency had "sufficient resources to remain open for a limited amount of time". He said further instructions would come if the shutdown lasts for more than a week.

More than half – 34,600 – of the Department of Transportation's 55,100 employees will continue working during a shutdown. The bulk of those staying on the job work for the Federal Aviation Administration, which operates the nation's air traffic control system.

Controllers and aviation, pipeline and railroad safety inspectors are among those who would continue to work.

At the National Highway Traffic Safety Administration, investigations on auto safety defects will be suspended, incoming information on possible defects from manufacturers and consumers won't be reviewed and compliance testing of vehicles and equipment will be delayed.