AKCAKALE    -    Turkey launched a military operation Wednesday against Kurdish fighters in northeastern Syria after U.S. forces withdrew from the area, with activists reporting airstrikes on a town on Syria’s northern border.

Turkish President Recep Tayyip Erdogan announced the start of the campaign, which followed an announcement Sunday by U.S. President Donald Trump that American troops would step aside in a shift in U.S. policy that essentially abandoned the Syrian Kurds. They were longtime U.S. allies in the fight against the Islamic State group.

“Our mission is to prevent the creation of a terror corridor across our southern border, and to bring peace to the area,” Erdogan said in a tweet.

He added that Turkish Armed Forces, together with the Syrian National Army, had launched what they called “Operation Peace Spring” against Kurdish fighters to eradicate what Erdogan called “the threat of terror” against Turkey

TV reports in Turkey said its warplanes had bombed Syrian Kurdish positions across the border. Turkish airstrikes hit he town of Ras al-Ayn on the Syrian side of the border, activists in Syria said.

Mustafa Bali, a spokesman for the U.S.-backed Kurdish-led Syrian Democratic Forces, said Turkish warplanes were targeting “civilian areas” in northern Syria, causing “a huge panic” in the region. There were no independent reports, however, on what was being struck in the initial hours of the operation.

Earlier Wednesday, warning of a “humanitarian catastrophe.” Syrian Kurdish forces who are allied with the United States issued a general mobilization call ahead of Turkey’s attack. The Turkish operation would ignite new fighting in Syria’s 8-year-old war, potentially displacing hundreds of thousands of people, and the Britain-based Syrian Observatory for Human rights reported that people had begun fleeing the border town of Tal Abyad. Kurdish politician Nawaf Khalil, who is in northern Syria, said some people were leaving the town for villages farther south.

Turkey has long threatened to attack the Kurdish fighters whom Ankara considers terrorists allied with a Kurdish insurgency in Turkey. Associated Press journalists on the Turkish side of the border overlooking Tal Abyad saw Turkish forces crossing into Syria in military vehicles Wednesday, although there was no official statement from either side that the offensive had begun.

Expectations of an invasion increased after Trump’s announcement on Sunday, although he also threatened to “totally destroy and obliterate” Turkey’s economy if the Turkish push into Syria went too far. Turkey has been massing troops for days along its border with Syria and vowed it would go ahead with the military operation and not bow to the U.S. threat. A senior Turkish official said Turkey’s troops would “shortly” cross into Syria, together with allied Syrian rebel forces to battle the Kurdish fighters and also IS militants.

Trump later cast his decision to pull back U.S. troops from parts of northeast Syria as fulfilling a campaign promise to withdraw from the “endless war” in the Middle East. Republican critics and others said he was sacrificing an ally, the Syrian Kurdish forces, and undermining Washington’s credibility.

Fahrettin Altun, the Turkish presidency’s communications director, called on the international community in a Washington Post op-ed published Wednesday to rally behind Ankara, which he said would also take over the fight against the Islamic State group. Turkey aimed to “neutralize” Syrian Kurdish militants in northeastern Syria and to “liberate the local population from the yoke of the armed thugs,” Altun wrote.

Erdogan discussed plans for the incursion with Russian President Vladimir Putin. Erdogan’s office said the Turkish leader told his Russian counterpart by phone that the planned military action in the region east of the Euphrates River “will contribute to the peace and stability” and also “pave the way for a political process” in Syria.

Turkey’s Defense Minister Hulusi Akar told the state-run Anadolu Agency that Turkish preparations for the offensive were continuing.