US Defence Secretary Jim Mattis resigned on Thursday, stating in a letter to President Donald Trump that he was stepping down because his world view was different from that of the president.

The US media reported that Secretary Mattis went to the White House on Thursday afternoon to persuade Trump to keep American troops in Syria but when he was rebuffed, he gave his resignation letter to the president.

Read: Trump declares victory over IS in Syria, orders US troops home

“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” Secretary Mattis wrote.

“We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances,” he added.

“Because you have the right to have a Secretary of Defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position.”

Secretary Mattis, however, said that he would stay till February 28 to allow sufficient time for a successor to be nominated and confirmed.

He wrote that he also wanted to ensure the change occurs well in advance of the transition of Chairman of the Joint Chiefs of Staff in September in order to ensure stability within the Defence Department.

Soon after Mattis left the White House, President Trump sent out two tweets, confirming that the Pentagon chief will leave at the end of February.

Various US media outlets reported that although Secretary Mattis differed with the president on several major issue, Trump’s decision to withdraw 2,000 American troops from Syria convinced him that now was the time to leave.

The reports claimed that Secretary Mattis and senior US generals were also upset with Trump for using his Twitter account even to announce a decision of huge global implications.

In Washington, Secretary Mattis’s experience and stability were widely seen as a balance to an unpredictable president and his departure would add to the chaos and unease that surrounds the Trump administration.

Trump, who is seen in Washington as an outsider, had included several four-star generals in his cabinet and proudly called them “my generals.” By the time Secretary Mattis’s resignation takes effect at the end of February, all of Trump’s generals would be gone.

CNN reported on Thursday evening that after President Trump’s Syria tweet, senior US defence officials were also bracing themselves for another surprise announcement: the withdrawal of US troops from Afghanistan.

The withdrawal, if announced, would have even greater impact on global affairs than the expected pullout from Syria.

Administration sources told CNN that President Trump had not yet made a final decision on Afghanistan, but officials “are concerned and convinced that he might do so, and soon.”

The US has about 14,000 troops in Afghanistan, to train, advise and assist Afghan forces.

“Any withdrawal would be complicated by the fact that the United States is part of NATO's Resolute Support mission,” CNN warned.