The US military has been ordered to begin planning to withdraw about half the troops in Afghanistan, a US defense official with direct knowledge of the matter told CNN on Thursday.

The official said planning is underway, and it could take months to withdraw the nearly 7,000 troops.

The decision was made Tuesday, at the same time as President Donald Trump's decision to withdraw the US military from Syria -- moves that precipitated Defense Secretary James Mattis' resignation announcement on Thursday.

The Wall Street Journal was first to report the plan for the Afghanistan withdrawal.

Several US defense officials told CNN that Trump wants to draw down US troops in Afghanistan. Two administration officials told CNN that Trump wants the plans drawn up in hopes he could announce the drawdown in his State of the Union speech, which is traditionally at the end of January or early February.

CNN reported earlier Thursday that officials throughout the administration were bracing themselves for Trump to make an announcement about the US presence in Afghanistan.

Multiple officials told CNN the military decisions were a factor in Mattis' decision to resign.

Gen. John Allen, a former commander of NATO and US forces in Afghanistan, told CNN on Thursday that a drawdown in Afghanistan would be a mistake.

"Pulling out right now, just the announcement would create chaos in the strategy," Allen said.

The US has about 14,000 troops in Afghanistan, most of which are present as part of a larger NATO-led mission 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.

Trump has long been critical of the US presence in Afghanistan, which began after the September 11, 2001, terrorist attacks. But lawmakers have echoed Allen's concern about a hasty departure.

South Carolina GOP Sen. Lindsey Graham told CNN's Kate Bolduan on Thursday that "according to our military commanders and everybody I know, we want to withdraw from Afghanistan with honor and do it based on conditions on the ground."

"Based on my assessment in Afghanistan, if we withdrew anytime soon, you would be paving the way for a second 9/11," Graham said.

Graham noted that Trump has said he wants other countries to do the fighting.

"Since August of 2017, 5,600 Afghans have died fighting the Taliban and ISIS," he told Bolduan. "Eighteen American killed in combat, four killed through accidents. God bless the 22."

Just two weeks before the news of the withdrawal plan, the chairman of the Joint Chiefs of Staff, Joseph Dunford, said at a Washington Post Live event that he "had not recommended" that the US withdraw.

"I had not recommended that we leave Afghanistan because, again, in my judgment, leaving Afghanistan not only would create instability in South Asia, but in my judgment would give terrorist groups the space within which to plan and conduct operations against the American people, our homeland and our allies," Dunford said. "And that really is the problem we are trying to solve."

The same day news broke of the Afghanistan decision, the Department of Defense released a report on "enhancing security and stability in Afghanistan."

The report cited a ceasefire, statements of religious scholars and recent talks with the Taliban as evidence of progress.

"The combination of military escalation and diplomatic initiative have made a favorable political settlement more likely than at any time in recent memory," the report said. (CNN)