Outgoing Defense Secretary Jim Mattis has signed orders to pull all American troops out of Syria in the coming weeks, a senior defense official told Fox News on Sunday.

Pentagon officials refused to discuss specifics including the timeline citing operational security for the roughly 2,000 U.S. troops in Syria.

Plans were set in motion hours after President Trump declared the Islamic State terror network “defeated” in Syria in a tweet Wednesday and ordered all American diplomats and soldiers out of the country.

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Trump pledged in campaign rallies last spring U.S. troops would be leaving Syria “very soon.”

While the U.S. military is planning to leave Syria, it has no plans to withdraw the more than 5,000 American troops in neighboring Iraq. Some of those special operations forces could be used for raids against “high value targets” in Syria from bases in Iraq if necessary, officials said.

The last ISIS stronghold in eastern Syria is the city of Hajin located not far from the Iraqi border. U.S. and allied artillery continue to fire on ISIS positions from Iraq. U.S.-led coalition air strikes also continue against the terrorist army in Syria.

But questions remain about what will happen once all American forces are out of Syria. “Challenges are ahead,” one U.S. official said. Currently, no Kurdish or Arab fighters in the U.S.-backed Syrian Democratic Forces (SDF) have the ability to call in American airstrikes.

It’s not clear whether the SDF will continue to receive arms and funding from the United States to sustain the fight against ISIS.

Trump said in a tweet Sunday the U.S. withdrawal from Syria would be “slow & highly coordinated,” after speaking to his Turkish counterpart, his second call with President Recep Tayyip Erdogan since Dec. 14. Fox News first reported it was Erdogan who convinced Trump to pull out of Syria.

Turkey’s president has called the Kurdish-dominated SDF a terrorist organization over its links to a Kurdish separatist group in Turkey which has killed thousands in recent decades.

In a separate tweet Sunday, President Trump said Mattis would be leaving by Jan. 1, forcing him out two months earlier than expected.

American military leaders have said the SDF is the best allied force on the ground in Syria to crush ISIS. Today, ISIS only controls one percent of the area it once held across Syria and Iraq — an area at its peak the size of Ohio — due in part to the U.S. military's alliance with the SDF in Syria.

Critics of the president’s decision to pull out of Syria said ISIS has not been defeated yet -- and lawmakers have urged Trump not to repeat the mistakes attributed to former President Obama when all U.S. troops were pulled out of Iraq in 2011, helping give rise to ISIS.

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Some officials have said the hundreds of American special operations forces in Syria had run out of targets and could be put back in if necessary. Others have said it would be a betrayal to the Kurdish and Arab fighters loyal to the U.S. military to leave Syria now. Hundreds of Kurdish fighters of the SDF have been killed fighting ISIS in Syria.

U.S. airstrikes began in Syria against ISIS in 2014 a year before any U.S. troops were on the ground.

Trump's envoy to the ISIS coalition, Brett McGurk, resigned Friday just one day after Mattis announced his retirement.

French and British special operations forces will remain in Syria, according to both countries defense ministries. Air strikes from those nations are set to continue.