Trump in two tweets claimed Turkey's Erdogan would "eradicate whatever is left" of the jihadist group Islamic State (IS) in Syria, targeted in a four-year multinational campaign and spearheaded inside northeastern Syria by Kurdish fighters backed by 2,000 US troops.

French President Emmanuel Macron late Sunday joined a chorus of NATO calls for a rethink, saying he "very deeply regrets the [US] decision made on Syria" and added that "an ally must be reliable."

Read more: US anti-IS envoy Brett McGurk also resigns

Kurdish affairs analyst Mutlu Civiroglu said the US withdrawal, announced abruptly by Trump last week, would open the way "for Turkey to start its operations against the Kurds, and a bloody war will begin."

Converging on Manbij?

Turkey's Anadolu news agency said Monday that troop reinforcements had been sent to Turkey's province of Kilis across the border from Manbij, a Kurdish-administered town in northern Syria where US troops are based.

Turkey-allied Syrian opposition forces were also massing near Manbij, said the Syrian Observatory for Human Rights.

Troops 'coming home'

A Kurdish-led alliance, SDF – backed until now by Washington in the anti-IS campaign - controls almost 30 percent of Syria. Kurdish People's Protection Units (YPG) within that alliance have long been viewed by Erdogan as a "terrorist offshoot" of long-standing Kurdish insurgency inside Turkey.

Trump, facing objectors even among his own Republican Party, retorted: "Our troops are coming home" and brought forward to January 1 the replacement of Jim Mattis, who quit as Pentagon chief, by his deputy Patrick Shanahan.

March 2018: US soldiers near Manbij, northern Syria

Mattis, a retired Marine Corps general, who also publicly criticized another Trump order to pull out up to half of the 14,000 US troops in Afghanistan, had been expected to remain until February.

Washington's order to quit Syria had been signed, a US military spokesman told the French news agency AFP.

Erdogan's office said "the two leaders 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."

Objections - even at Defense Department

Trump's plan was not popular at the Department of Defense, admitted White House Chief of Staff Mick Mulvaney, who is acting as replacement for John Kelly, another disaffected general once recruited by Trump.

Read more: Germany, France, UK say IS not defeated

On American ABC television, Mulvaney said Mattis and Trump "just could never get on the same page" on Syria, adding that Trump would not change his mind.

Mulvaney: Trump, Mattis never 'on the same page'

Trump, since his 2016 presidential campaign had said "he wanted to get out of Syria," said Mulvaney, adding that the president "is entitled to have a secretary of defense who is committed to that same end."

Also on ABC; Senate Majority Leader Mitch McConnell joined leading Republicans on foreign affairs in urging Trump to withhold a final decision for 90 days, saying withdrawal would be "a premature and costly mistake."

In September the US Congressional Research Service said despite the multinational coalition's liberation of "most of the territory" formerly held by the jihadists in Syria and Iraq the "IS leadership remains at large and IS fighters appear to be evolving into an insurgent force."

IS controlled large areas of Syria and Iraq from 2014 until 2017.

Israel warns Iran

Israeli Prime Minister Benjamin Netanyahu on Sunday warned Iran as Syria's southeastern neighbor and backer of Lebanon-based Hezbollah forces not to exploit Trump's decision.

"We will continue to act against Iran's attempt to establish a military presence in Syria, and if the need arises, we will even expand our activities there," Netanyahu said.

Former Israeli national security adviser Yaakov Amidror said: "From now on, it will be a free ride for the Iranians and they will use the corridor [through Syria] logistically to enhance their capabilities to build the military forces in Syria and to help Hezbollah afterwards," Amidror said.

ipj/kms (AFP, dpa, AP)