United States President Donald Trump has announced a replacement for Defence Secretary Jim Mattis who will start two months earlier than had been expected.

Key points: Patrick Shanahan will take on the role in an acting basis from January 1

Patrick Shanahan will take on the role in an acting basis from January 1 General Mattis's resignation letter expressed that he did not align with Mr Trump's foreign policy

General Mattis's resignation letter expressed that he did not align with Mr Trump's foreign policy A White House official told Reuters that Mr Trump does not want the transition to drag out

According to a senior White House official, the move to bring forward the replacement was sparked by Mr Trump's anger at General Mattis's resignation letter and its public rebuke of his foreign policy.

Mr Trump said Deputy Defence Secretary Patrick Shanahan would take over for General Mattis on an acting basis on January 1.

In a tweet, Mr Trump called the former Boeing Co executive "very talented".

"He will be great!" Mr Trump said about Mr Shanahan.

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General Mattis abruptly said he was quitting on Thursday after falling out with Mr Trump over his foreign policy, including surprise decisions to yank troops from Syria and start planning a drawdown in Afghanistan.

General Mattis distributed a candid resignation letter addressed to Mr Trump that laid bare the growing divide between them, and implicitly criticised Mr Trump for failing to value America's closest allies, who fought alongside the US in both conflicts.

He said Mr Trump should have a defence secretary more aligned with his views.

Speaking to Reuters, the senior White House official said Mr Trump was irked by the attention given to General Mattis's resignation letter.

"He just wants a smooth, more quick transition and felt that dragging it out for a couple of months is not good," the official said, on condition of anonymity.

The official said Mr Trump was expected to pick a permanent nominee for defence secretary over the next couple of weeks.

General Mattis, a retired Marine who was highly regarded by Republicans and Democrats, has far wider political support in Washington than Mr Trump himself.

General Mattis "will continue to focus on what he needs to do … to ensure a smooth transition", a US defence official said, also speaking on condition of anonymity.

Trump flags 'slow' Syria withdrawal

There are currently around 2,000 US troops in Syria. ( Reuters: Rodi Said )

In a shock announcement on Wednesday, Mr Trump said he was withdrawing US troops from Syria, a decision that upended American policy in the region.

General Mattis had advised against the Syria withdrawal — pointing to it as one of the factors in his resignation.

Tweeting on Sunday, Mr Trump said he had spoken with Turkish President Tayyip Erdogan about a "slow and highly coordinated" withdrawal, suggesting that he might slow down the process after the barrage of criticism.

"We discussed [the Islamic State terrorist group], our mutual involvement in Syria, and the slow and highly coordinated pull-out of US troops from the area," Mr Trump said in a tweet.

A US military official, speaking on the condition of anonymity, said that the withdrawal would be "safe, professional and deliberate", but was not aware of any new guidance from the White House.

A plan on the pull-out is expected to be presented by commanders to the Pentagon some time this week, the official said.

But despite Mr Erdogan's assurance that Turkey would postpone a planned military operation as a result of the US announcement, witnesses reported that large amounts of Turkish troops were gathering at the Syrian border.

A Reuters witness saw hundreds of vehicles head to the southern border province of Kilis after leaving Hatay, another border province.

The convoy included tanks, howitzers, machine guns and buses carrying commandos, the witness said.

Footage from broadcaster TRT World showed some Turkish convoys entering Syria via the Turkish border town of Karkamis, which is located some 35 kilometres north of the northern Syrian town of Manbij.

The convoys are crossing into an area controlled by the Free Syrian Army (FSA), a Turkish ally, and are heading to the frontlines of Manbij, TRT World said.

French President Emmanuel Macron said on Sunday he deeply regretted Mr Trump's Syria decision.

"I very deeply regret the decision made on Syria," Mr Macron said during a news conference in Chad.

"To be allies is to fight shoulder to shoulder. It's the most important thing for a head of state and head of the military.

"An ally should be dependable."

Reuters