WASHINGTON (REUTERS) - United States President Donald Trump on Sunday (Dec 23) said he was replacing Defence Secretary Jim Mattis two months earlier than had been expected, a move officials said was driven by Mr Trump's anger at Mr Mattis' resignation letter and its rebuke of his foreign policy.

Last Thursday, Mr Mattis abruptly said he was quitting, effective Feb 28, after falling out with Mr Trump over his foreign policy, including surprise decisions to withdraw all troops from Syria and start planning a drawdown in Afghanistan.

Mr Trump has come under withering criticism from fellow Republicans and international allies in recent days over his moves to wind down US involvement in Syria and Afghanistan, against the advice of his top aides and US commanders.

The exit of Mr Mattis, highly regarded by Republicans and Democrats alike, added to their concern over what they see as Mr Trump's unpredictable, go-it-alone approach to global security.

In announcing his resignation, Mr 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.

Mr Trump, who tweeted last Thursday that Mr Mattis was "retiring, with distinction, at the end of February", made his displeasure clear last Saturday by tweeting that the retired Marine general had been "ingloriously fired" by former president Barack Obama and he had given him a second chance.

Mr Obama removed Mr Mattis as head of US Central Command in 2013 because of what officials at the time said were perceived to be his hawkish views on Iran.

Last Thursday's tweet was dictated to an aide to send before Mr Trump read Mr Mattis' resignation letter, a senior administration official told reporters on Sunday.

"That's not the kind of letter of resignation I think you should write," the official said, adding Secretary of State Mike Pompeo had told Mr Mattis on Sunday he would be leaving on Jan 1.

On Sunday, Mr Trump tweeted that Deputy Defence Secretary Patrick Shanahan would take over for Mr Mattis on an acting basis on Jan 1. In a tweet, Mr Trump called the former Boeing Co executive "very talented".

In his letter, Mr Mattis had said he would step down at the end of February to allow for a successor to be confirmed and attend Congressional hearings and a key Nato meeting. A senior White House official said that Mr Trump was irked by the attention given to Mr Mattis' 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 nominee for defence secretary over the next couple of weeks. Defence Department spokesman Dana White said vMattis would work with Mr Shanahan and Pentagon leadership to ensure the department "remains focused on the defence of the nation during this transition".

Mr Shanahan, in his job as deputy defence secretary, had largely focused on internal Pentagon reform and issues like the creation of a Space Force, a project championed by Mr Trump but which has met resistance from lawmakers and parts of the Pentagon.

A senior administration official told Reuters that Mr Shanahan "has a deep-seated understanding of military operations, and global security affairs, and importantly, has the breadth of large-scale business management experience that will enable him to effectively oversee the Defence Department".

'SLOW' SYRIA PULLOUT

In a shock announcement last Wednesday, Mr Trump said he was withdrawing US troops from Syria, citing its cost both in terms of lives of US military and financially.

A day later, American officials told Reuters the US was planning on pulling about half of the 14,000 US troops in Afghanistan.

Mr Mattis, a retired Marine general whose embrace of Nato and America's traditional alliances often put him at odds with Mr Trump, had advised against the Syria withdrawal - one of the factors in his resignation.

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

"We discussed ISIS, our mutual involvement in Syria, and the slow and highly coordinated pullout 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 pullout is expected to be presented by commanders to the Pentagon some time this week, the official said.

French President Emmanuel Macron said on Sunday that he deeply regretted Mr Trump's Syria decision. The plan has also prompted unusually sharp criticism of Mr Trump from some of his fellow Republicans.

Senator Bob Corker, the influential Republican chairman of the Senate Foreign Relations Committee, said he was "saddened" by Trump's withdrawal decision.

"I think he knows he made a mistake," Mr Corker, who is retiring, said on CNN. "The president's tendencies are to dig in and double-down if he knows he has done something that is probably incorrect."