US Defence Secretary Jim Mattis will leave his post on January 1, two months ahead of schedule, after an angry President Donald Trump announced his early replacement.

The announcement that deputy Pentagon chief Patrick Shanahan will take over as acting secretary to cover Mr Mattis’ accelerated departure comes as US congress reportedly issued an ­extraordinary declaration on the importance of the Australia-US alliance.

Mr Mattis’ decision to stand down, accompanied by a scathing resignation letter, came after Mr Trump rejected Pentagon advice and announced the withdrawal of US troops from Syria.

The unilateral decision shocked both sides of US politics and caused anxiety among US allies around the world.

Mr Mattis originally said he would stay through February to ensure an orderly transition, but officials said the fallout of his decision to leave – including the shock and dismay expressed on Capitol Hill – angered Mr Trump and likely led to him leaving earlier than planned.

In a stunning resignation letter, Mr Mattis made clear he did not see eye-to-eye with a President, who has expressed disdain for NATO and doubts about keeping troops in Afghanistan.

Members of the US congress, including 10 Republicans and 11 Democrats, moved to allay Australian “concerns’’ about the retired generals departure, Canberra’s “best friend” in Washington.

Mr Mattis made regular visits to the Pacific region and was a vocal supporter of Australia’s military contributions to the US alliance.

Mr Mattis’ resignation was followed Sunday by the departure of Brett McGurk, US envoy to the international coal­itio­n of nations fighting Islamic State.

In a letter obtained by News Corp, the politicians delivered a thinly veiled broadside against Mr Trump and declared they would act to ­protect the “special bond” ­between the US and Australia.

The letter, addressed to Australia’s US ambassador, Joe Hockey, was quoted by News Corp as saying: “We share many Australians’ concerns about Secretary of Defence Jim Mattis’ recent announcement that he will step down from the position in February 2019”.

“We write to you to reaffirm our commitment to the enduring alliance between the United States and Australia,” it said.

Lieutenant Colonel Joseph Buccino, a spokesman for Mr Shanahan, said the former Boeing executive would accept the appointment as acting secretary.

“[The] deputy secretary will continue to serve as directed by the President, and the Department of Defence will remain focused on the defence of the nation,” Mr Buccino said on Monday morning (Australian time).

While Mr Mattis’s resignation followed Mr Trump’s announcement he would pull troops out of Syria, Washington officials told Reuters the resignation was the result of an accumulation of disagreements.

On Sunday, Mr Trump’s acting chief of staff said the President had known for “quite some time now” that he and Mr Mattis “did not share some of the same philosophies … have the same world view”.

Mick Mulvaney told US broadcaster ABC that the President and his defence chief “just could never get on the same page” on Syria, adding Mr Trump had said since his presidential campaign that “he wanted to get out of Syria”.

Mr Mulvaney said the Mr Trump “is entitled to have a secretary of defence who is committed to that same end”.

Asked whether Mr Trump wanted a Pentagon leader willing to challenge him or someone in lock step with his views, Mr Mulvaney said “a little bit of both”.

“I’ve encouraged [Mr Trump] to find people who have some overlap with him, but don’t see the world in lockstep with him,” he said.

Meanwhile, Senate majority leader Mitch McConnell joined leading Republicans on foreign affairs in urging Mr Trump to reconsider his decision to withdraw American forces from Syria.

Mr Mulvaney, asked on ABC whether there was any chance the President might change his mind on the Syria decision, said: “No. I think the president has told people from the very beginning that he doesn’t want us to stay in Syria forever.

“You’re seeing the end result now of two years’ worth of work. But keep in mind it’s not unusual for a president to lose members of the cabinet over these types of disagreements.”

-with AAP