Trump announced Wednesday that the roughly 2,000 US troops would leave civil war-racked Syria, where they have been deployed to assist in a multinational fight against the Islamic State (IS) jihadist group.

"The execute order for Syria has been signed," a US military spokesperson told AFP on Sunday in response to a query, without providing further details.

Turkey was a rare ally that lauded Trump's decision on Syria, a country where it will now have a freer rein to target Kurdish fighters who were armed and trained by the US and played a major role in the war against IS but are deemed terrorists by Ankara.

- 'We've won' -

Trump and President Recep Tayyip Erdogan spoke by telephone on Sunday and "agreed to ensure coordination between their countries' military, diplomatic and other officials to avoid a power vacuum which could result following any abuse of the withdrawal and transition phase in Syria," the Turkish presidency said in a statement.

Trump tweeted that he and Erdogan "discussed (IS), our mutual involvement in Syria, & the slow & highly coordinated pullout of U.S. troops from the area."

White House aides cited by The Washington Post said Trump's advisors have persuaded him to withdraw the troops more slowly than he would like in order not to jeopardize their safety.

A withdrawal could have extraordinary geopolitical implications, and it runs counter to long-established US policy for the region.

Thousands of IS jihadists are thought to remain in Syria, but Trump on Wednesday declared that "we've won against ISIS," using another acronym for the extremists.

Late Sunday he tweeted that Erdogan had assured him that any IS fighters remaining will be eliminated.

"President @RT_Erdogan of Turkey has very strongly informed me that he will eradicate whatever is left of ISIS in Syria," Trump said in a Tweet.

- An already devastated region -

Repeating a pattern of admiring comments towards global strongmen, Trump added that Erdogan "is a man who can do it."

The US president concluded: "Our troops are coming home!"

US politicians -- including those from his own Republican party -- and international allies fear the withdrawal is premature and would further destabilize the already devastated region.

A US pullout, said Mutlu Civiroglu, a Kurdish affairs analyst, will open the way "for Turkey to start its operations against the Kurds, and a bloody war will begin."

French President Emmanuel Macron on Sunday said he "deeply regretted" Trump's decision, and that "an ally must be reliable."

Israel has been careful to point out that it respects the US decision, but analysts say that beneath those public pronouncements are concerns over whether its main enemy Iran will have a freer hand.

US troops will leave under the auspices of a new Pentagon chief set to start next month, after Jim Mattis resigned from the post citing major differences, including on Syria, with the often-impulsive Trump.

Several US politicians from both parties rejected Trump's claim that IS had been defeated. The decision also caused alarm and dismay in the US military over the prospect of suddenly abandoning Washington's Kurdish partners.

- Turmoil -

Trump's sudden decision sparked turmoil within his administration, prompting the resignation of Brett McGurk, the special envoy to the anti-IS coalition, as well as Mattis.

Plans for the troop pullout will now be overseen by Deputy Secretary of Defense Patrick Shanahan, who Trump on Sunday said would replace Mattis starting January 1.

Mattis, 68, had said he would leave at the end of February to allow a smooth transition for the next chief of the US military -- but a reportedly angry Trump accelerated his departure by two months.

"And now Trump gets rid of SecDef Mattis almost immediately. No smooth transition. No effort at reassurance to allies. Just vindictive," Carl Bildt, a former Swedish prime minister and now co-chair of the European Council on Foreign Relations, said on Twitter.

According to US media, Trump voiced resentment over news coverage of Mattis' stinging resignation letter that laid bare his fundamental disagreements with the president.

Days later, special envoy McGurk made a similar move, saying he could not support Trump's Syria decision that "left our coalition partners confused and our fighting partners bewildered."

Unlike Mattis, Shanahan has never served in the military and has spent most of his career in the private sector, including with aircraft giant Boeing.

Until Trump finds a permanent Pentagon chief, Shanahan will lead plans for US troops to leave Syria along with a significant drawdown in Afghanistan, both of which critics worry will leave war-torn regions at risk of continued and potentially heightened bloodshed.

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