The U.S. pullout will leave the oil-rich eastern third of Syria up for grabs. It is currently controlled by Kurdish-led forces that the Americans have backed over the past four years, with multiple parties seeking to move in. They now face a triple threat from the Syrian government, IS and Turkey, which views them as terrorists because of their links to a Kurdish insurgent group inside Turkey.
Youssef Hammoud, spokesman for Turkey-backed Syrian opposition groups, said they have up to 15,000 trained fighters ready to deploy alongside Turkish forces, and they are already preparing to move into Manbij - a Kurdish-administered town in northern Syria where U.S. troops are based.
Hammoud said there is "no alternative" to Turkish forces and their allies replacing U.S troops.
"We are ready to fight Daesh," said Hammoud, using the Arabic term for the Islamic State group, though IS militants are largely confined to a remote desert enclave hundreds of miles (kilometers) to the southeast of Manbij.
U.S. President Donald Trump announced last week that the U.S. with withdraw all of its 2,000 forces in Syria, a move that will leave control of the oil-rich eastern third of Syria up for grabs.
Ilham Ahmed, a Syrian Kurdish official, said the Kurdish-led forces are now reaching out for potential new allies, underscoring the dire situation the group now finds itself in.
"We will deal with whoever can protect the ... stability of this country," said Ahmed.
Ahmed said her forces are talking with the Russians and the Syrian government - both rivals of the United States - as well as European countries about ways to deal with the U.S. withdrawal. She didn't elaborate.
Abandoned by the U.S., the Kurdish militia are confronting the dilemma of whether to try to hold on to the 30 percent of Syria they wrested from IS. The territory includes some of the richest oil fields in north and east Syria but also is home to large Arab populations.
The Kurds could pull back to the Kurdish-majority region in the far northeast but that would leave resources and Kurdish-majority pockets in the east isolated and vulnerable.
The militia could also negotiate with Damascus, allowing a return of government forces back into the east in hopes of gaining a level of self-rule for Kurds. The government has so far rejected the notion of such autonomy
Syrian government forces have reportedly been massing troops in Deir el-Zour province, across the Euphrates River from Kurdish-held territory.
On Monday, Iraq said it could consider deploying troops inside Syria to protect Iraq from threats across its borders. Iraq's Prime Minister Adel Abdul-Mahdi said his government is "considering all the options."
U.S. President Donald Trump has said the withdrawal from Syria will be slow and coordinated with Turkey, without providing a timetable. Turkey said the two countries will ensure there is no "authority vacuum" once the U.S. troops leave.
A Pentagon spokesman, Navy Cmdr. Sean Robertson, said the execute order for withdrawal has been signed but provided no further details.
Turkish Presidential spokesman Ibrahim Kalin said a U.S. military delegation is expected in Turkey this week.
Turkey says it and its Syrian Arab allies can replace the U.S. in preventing a resurgence of the Islamic State group.
Kalin said there will be no "step back, weakness, halting or a slowing down" of the fight against the IS. Turkey has made clear it will not tolerate a contiguous Kurdish-held enclave along its border with Syria.
Hammoud, spokesman for the Turkey-backed Syrian opposition forces, said their fighters and weapons were deploying on the front line with Manbij, a Kurdish-administered town in northern Syria where U.S. troops are based. They are preparing to take Manbij first, he said.
Manbij was at the center of an agreement the U.S. and Turkey reached in June under which Kurdish forces were to withdraw. In recent weeks Turkey said the U.S. was dragging its feet in implementing the deal and vowed to launch a new offensive against the Kurds.
Those threats and a phone call with Turkish President Recep Tayyip Erdogan last week appear to have triggered Trump's decision to withdraw the U.S. forces based in Syria.
Kurdish forces in Manbij "have taken measures to fend off any attack," said the spokesman for the Kurdish-led Manbij Military Council, Sharfan Darwish.
Trump has claimed to have defeated IS, but the Kurdish fighters are still battling the extremists in the remote town of Hajin near the Iraqi border. The Britain-based Syrian Observatory for Human Rights said the fighting displaced nearly 1,000 civilians on Sunday alone.
Ahmed, the senior Syrian Kurdish official, had just returned from a trip to France in which she called on Paris to play a larger role in Syria following the U.S. withdrawal.
"I urge Trump to go back on his decision inciting Erdogan against the Syrian people in general and the Kurdish people in specific," she said.
In the fight against IS, hundreds of Kurdish fighters were killed. "I call on him to return the favor."
Associated Press writer Suzan Fraser in Ankara, Turkey, and Philip Issa in Baghdad contributed to this report.
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