Turkey launched a military ground incursion into northeastern Syria to force U.S.-backed Kurdish militants controlling the border area away from the region, the Defense Ministry said, after President Donald Trump ordered U.S. forces to stand aside.

Turkish armored vehicles and tanks crossed into Syria after Turkish F-16s and artillery units targeted positions of Kurdish YPG militants earlier Wednesday. The Turkish military carried out the incursion together with allied Syrian rebels in an effort to seize areas to the south of the frontier towns.

President Recep Tayyip Erdogan earlier announced the start of the operation, code-named “Peace Spring,” on Twitter and said it would also target Islamic State. Russia, Iran and some top European officials urged Turkey to act with restraint. The conflict has sparked concerns that renewed chaos in Syria could lead to a jihadist resurgence and push the Kurds -- America’s allies in the fight against IS -- into the arms of President Bashar al-Assad.

Turkey has battled Kurdish separatists for years and had repeatedly warned it would not allow the creation of a Kurdish proto-state on its immediate border. Once it seizes the area, Turkey plans to resettle 2 million Syrian refugees, most of them Arabs, in the border zone, further complicating a combustible situation.

A small forward group of Turkish forces first entered Syria early Wednesday at two points close to the towns of Tal Abyad and Ras al-Ayn, according to a Turkish official, who spoke on condition of anonymity. Turkish planes and shells pounded the towns as the incursion began. Residents fled though Kurdish forces had vacated positions before the attack, which is expected to involve tens of thousands of soldiers backed by tanks and armored personnel carriers from NATO’s second-largest army.

The Turkish lira weakened as the conflict started.

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Turkey’s advance followed a dramatic reversal of U.S. policy. Trump told Erdogan in a phone call on Sunday that dozens of American troops who’d been working closely with Kurdish forces in the fight against Islamic State would pull back, effectively clearing the way for a Turkish incursion.

Asked by reporters Wednesday at the White House what he’d do if Erdogan wipes out the Kurds, Trump said: “I will wipe out his economy if that happens,” adding that he hopes the Turkish leader “will act rationally.”

Trump’s announcement Sunday appeared to surprise allies at home and abroad. Republican Senator Lindsey Graham, normally a staunch Trump ally on Capitol Hill was among several Republicans angered by the move. He tweeted Wednesday: “Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration. This move ensures the reemergence of ISIS.”

The Kurdish-led SDF said they would defend their “own people,” potentially relegating the battle against Islamic State.

The Kurdish YPG militia that forms the backbone of the SDF has been one of America’s closest partners in the fight against Islamic State and is holding thousands of jihadist fighters and their families in camps and detention centers in northeastern Syria.

The head of the SDF, Mazloum Abdi, wrote Trump on Wednesday and implored him to intervene.

“We still hope that you will prevent the coming catastrophe against the Kurdish people and other peoples in the region as a result of the Turkish invasion,” Abdi wrote in the letter, which praised earlier cooperation between the SDF and U.S. forces.

While Trump said Turkey would become responsible for the detainees, who include foreign fighters from Europe, it was not clear if there was a mechanism in place to transfer them to Turkish custody. Trump was criticized at home for a decision that could see Islamic State fighters escape or regroup.

Analysts said a U.S. pullback could ultimately play into the hands of Russia, whose military intervention helped turn the tide of the Syrian civil war in favor of Assad. As the Turkish offensive got underway, the Associated Press reported that the YPG had asked Russia to mediate talks between them and the Assad government.

Turkey sees the YPG as a threat due to its link to the separatist PKK, another Kurdish group the Turkish government been battling for decades. It’s considered a terrorist organization by the U.S. and the European Union.

Its offensive into northern Syria first aims to surround towns in a strip of border territory, before pushing further south in an effort to dismantle any chance of a Kurdish state emerging on its doorstep, according to two Turkish officials, who requested anonymity to discuss sensitive military planning.

The first targets will be the Syrian towns of Kobani, Tal Abyad and Ras al-Ayn, all held by the YPG and located along the former Berlin-Baghdad railway that for hundreds of miles forms the frontier with Turkey, according to the officials.

The military aims to penetrate at least 30 kilometers (19 miles) deep into Syrian territory and secure the M-4 highway that runs parallel to the frontier all the way to Iraq in the east, they said. Erdogan was keen to act before winter set in and made it difficult for tanks to operate in muddy terrain.

“What aggravates the operational risks is the deep-running mistrust between Turkey and the U.S.,” said Nihat Ali Ozcan, a strategist at the Economic Policy Research Foundation in Ankara. “Turkey is very much worried about whether the U.S. will share intelligence with the YPG over Turkish troop positions to help them defend themselves.”

Turkish commanders expect to be confronted by a sophisticated foe after the battle-hardened YPG were armed by the U.S. and other Western militaries to help fight Islamic State, they said. Erdogan has chastised Washington for backing the Kurds, but he only pushed ahead with the operation after Trump reversed years of U.S. policy.

“YPG militants have two options: They can defect or we will have to stop them from disrupting our counter-ISIS efforts,” Fahrettin Altun, Erdogan’s chief of communications, wrote on Twitter early Wednesday.

--With assistance from Lin Noueihed, Taylan Bilgic, Glen Carey and Tony Capaccio.