File photo shows the U.S. Defense Secretary James Mattis at the Pentagon, Virginia, the United States, on Aug. 7, 2018. (Xinhua/Liu Jie)

WASHINGTON, Dec. 23 (Xinhua) -- U.S. President Donald Trump announced Sunday that he has picked Deputy Secretary of Defense Patrick Shanahan as the acting Pentagon chief, forcing outgoing James Mattis to step down earlier than planned.

Shanahan will assume the role starting Jan. 1, 2019, Trump said in a tweet Sunday noon.

"Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!" The president added.

The announcement came only days after Trump announced that Mattis, the current defense secretary, will retire at the end of February next year.

Shortly before Trump tweeted his decision on Sunday, news broke out that the president was furious at days of negative news coverage over Mattis' resignation and his rebuke over the administration's treatment of U.S. allies in a resignation letter.

"One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships," Mattis wrote Thursday.

The 68-year-old former Marine Corps general said that his views on treating the allies with respect are strongly held and informed by over four decades of immersion in these issues.

"Because you have a right to have a Secretary of Defense whose views are better aligned with yours" on many subjects, "I believe it is right for me to step down from my position," Mattis added.

Mattis offered his resignation a day after Trump ordered the withdrawal of the 2,000 or so U.S. troops from Syria and declared victory over the Islamic State militant group, a move that Mattis reportedly disagreed strongly.

The scope of Mattis' divergence with Trump went far beyond the U.S. military mission in Syria.

Mattis is a defender of the U.S. alliance with the North Atlantic Treaty Organization (NATO), while the president once bashed NATO for being "obsolete" and often scolded member countries for failing to contribute sufficient funds.

The two also disagreed on issues such as banning transgender recruits from the military, canceling the joint U.S.-South Korea military exercises, pulling the United States out of the Iran nuclear deal, as well as deploying troops to the U.S.-Mexico border.

"When President Obama ingloriously fired Jim Mattis, I gave him a second chance," Trump tweeted Saturday. "Some thought I shouldn't, I thought I should. Interesting relationship - but I also gave all of the resources that he never really had. Allies are very important - but not when they take advantage of U.S."

During the Obama administration, Mattis was forced out as head of U.S. Central Command in 2013 over differences about handling Iran.

Mattis' leaving the Trump administration earlier means that he will not oversee U.S. troops withdrawn in Syria as well as reported reduction of U.S. soldiers in Afghanistan.

Seen by his supporters as the only remaining force of stability in the Trump administration, Mattis was the latest in a slew of cabinet members and administration officials who either resigned or were forced to quit within the last two years.

Brett McGurk, the U.S. envoy for the global coalition to defeat Islamic State, also resigned earlier in the past week, another sign of senior officials' objection over the U.S. troops withdrawal from Syria.

Trump said Thursday Mattis would "be retiring, with distinction", but did not mention him in announcing the replacement.

Shanahan, 56, became deputy secretary of defense in July 2017. The former longtime Boeing executive reportedly has strong relationships with both Trump and Vice President Mike Pence but is set to face questions over his lack of foreign policy or government experience.

A spokesperson for Shanahan said he will accept the appointment as acting secretary.

"Deputy Secretary will continue to serve as directed by the president, and the Department of Defense will remain focused on the defense of the nation," the spokesperson said on Sunday.

A spokeswoman for Mattis, Dana White, said on Twitter that the outgoing secretary will focus over the next week on ensuring a smooth transition.

The reshuffle at the Pentagon also comes as parts of the U.S. federal government are shut down over a budget impasse.