“Ships and submarines will remain at sea, our aircraft will continue to fly and our warfighters will continue to pursue terrorists throughout the Middle East, Africa and South Asia,” Mattis wrote in the memo to all defence personnel.

“Steady as she goes – hold the line. I know our Nation can count on you.”

Mattis even joked during an event in Washington on Friday that “the submarine that put to sea last week will still be put to sea for three months and God bless them, the lads will not have any email connectivity” so they won’t know what lawmakers are up to.

His sanguine comments conflicted with those of Trump, Vice-President Mike Pence and some Republican lawmakers who said the shutdown was harming US ability to protect its citizens. “Democrats are holding our Military hostage over their desire to have unchecked illegal immigration. Can’t let that happen!” Trump tweeted Saturday.

“When Democrats start paying our armed forces and first responders we will reopen negotiations,” Press Secretary Sarah Huckabee Sanders said in a statement late Friday that termed Democrats “obstructionist losers”.

Pence, speaking to US troops Saturday during a brief refuelling stop in Ireland on his way to the Middle East, also took up the cudgel.

Democrats “put politics ahead of our national defence, put politics ahead of meeting the obligations of our national government,” he said.

The vice-president tweeted photos showing him meeting troops on their way to Kuwait for a six-month deployment.

“It’s disappointing to every American that Democrats in the Senate would shutdown the gov’t when we have troops in harms way,” he said on Twitter.

Representative Martha McSally, an Arizona Republican running for the Senate, said Democrats chose “illegals over our American troops” and that more than 2 million members of the military won’t get a pay cheque as a result.

Representative Mark Meadows of North Carolina tweeted that “our brave military men and women deserve better”.

The US government entered a partial shutdown Saturday as Senate Democrats and a handful of Republicans blocked a bill to fund the government after the two parties failed to break their deadlock over immigration. Negotiations were continuing Sunday.

Democrats want legislation to protect undocumented immigrants brought to the US as children, and Republicans are setting conditions for border security and enforcement.

But Democrats didn’t ignore the possible military impact of the shutdown they blame on Trump and his allies in Congress.

Senate Democratic Leader Chuck Schumer said the military “has to be given the certainty it needs” with longer-term spending legislation. And his House counterpart Nancy Pelosi cited earlier comments by Mattis about budget uncertainty hurting the military.

While active troops would remain on the job, reserve training operations would be curtailed and as much as 50 per cent of the military’s civilian workforce could be furloughed, Mattis said.

Paycheques could also be disrupted if the shutdown drags on.

Active military, reserve and national guard members would only be paid for money earned before January 20, according to the Pentagon.

The Air Force Academy, which uses appropriated funds to pay for its sports programmes, cancelled all athletics activities until further notice, including men’s and women’s basketball matchups.

Military leaders hope to benefit from an eventual spending deal for the rest of fiscal 2018 that lawmakers are still negotiating.

Congressional Republican leaders want to boost defence spending around US$74 billion over the statutory cap of US$549 billion set under the 2011 Budget Control Act.

Mattis said Friday that, in general, delays over the budget have been damaging, although he didn’t tie that specifically to the latest machinations in Congress.

“No enemy in the field has done more to harm the readiness of the US military than” the “budgetary confusion” imposed by the caps and by stopgap spending measures passed by Congress, Mattis said.

The negotiations have been slowed down, in part, by Democrats’ demand that spending for non-military domestic programmes be increased by as much as Pentagon funding. Conservatives have opposed increases in domestic spending.