AKCAKALE, Turkey (Reuters) - Turkish troops and their Syrian rebel allies attacked Kurdish militia in northeast Syria on Wednesday, pounding them with air strikes and artillery before starting a cross-border ground operation that could transform an eight-year-old war.

The assault began days after U.S. President Donald Trump pulled American troops out of the way, prompting denunciations from senior members of his own Republican Party who say he abandoned the Syrian Kurds, loyal allies of Washington.

“The Turkish Armed Forces and the Syrian National Army have launched the land operation into the east of the Euphrates river as part of the Operation Peace Spring,” the Turkish defense ministry tweeted after nightfall, following a day of pounding the area from the air.

Turkish media reported troops entering Syria at four points, two of them close to the Syrian town of Tel Abyad and two close to Ras al Ain further east.

Turkey told the United Nations Security Council in a letter seen by Reuters that its military operation would be “proportionate, measured and responsible.” The 15-member body will meet on Thursday to discuss Syria at the request of the five European members, Britain, France, Germany, Belgium and Poland.

Thousands of people fled Ras al Ain toward Hasaka province, held by the Kurdish-led Syrian Democratic Forces (SDF). The Turkish air strikes killed at least five civilians and three fighters from the SDF and wounded dozens of civilians, the SDF said.

Reuters journalists at Akcakale on the Turkish side of the frontier watched as explosions struck Tel Abyad. After dark, the red flare of rockets could be seen fired across the border into Tel Abyad, and flames burned near the town. Explosions from Tel Abyad could be heard eight hours into the bombardment. A witness reached by telephone said civilians were fleeing en masse.

SDF fighters repelled a ground attack by Turkish troops in Tel Abyad, SDF spokesman Mustafa Bali said on Twitter.

The assault on the Kurds - for years Washington’s main allies on the ground in Syria - is potentially one of the biggest shifts in years in the Syrian war that has drawn in global and regional powers. The Kurds played a leading role in capturing territory from Islamic State, and now hold the largest swathe of Syria outside of the hands of President Bashar al-Assad.

Russia, Assad’s strongest foreign ally, urged dialogue between Damascus and Syria’s Kurds.

Trump’s decision to pull forces out of the way was denounced by some Kurds as a “stab in the back”.

‘BAD IDEA’

Trump called the Turkish assault a “bad idea” and said he did not endorse it. He expected Turkey to protect civilians and religious minorities and prevent a humanitarian crisis, he said.

But one of Trump’s closest fellow Republican allies, Senator Lindsey Graham, said failing to support the Kurds would be “the biggest mistake of his presidency”.

Representative Liz Cheney, a Republican hawk, said: “The U.S. is abandoning our ally the Kurds, who fought ISIS (Islamic State) on the ground and helped protect the U.S. homeland. This decision aids America’s adversaries, Russia, Iran, and Turkey, and paves the way for a resurgence of ISIS.”

Turkish President Tayyip Erdogan, announcing the start of the action, said the aim was to eliminate what he called a “terror corridor” on Turkey’s southern border.

European and Arab countries called on Ankara to halt.

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Turkey had been poised to enter northeast Syria since the U.S. troops who had been fighting alongside Kurdish-led forces against Islamic State started to leave.

A Turkish security source told Reuters the military offensive, dubbed “Operation Peace Spring”, opened with air strikes. Turkish howitzer fire then hit bases and ammunition depots of the Kurdish YPG militia. Turkey says the YPG, the main component of the U.S.-backed SDF, is a terrorist group linked to Kurdish insurgents that have fought in Turkey for years.

The artillery strikes, which also targeted YPG gun and sniper positions, were aimed at sites far from residential areas, the Turkish source said.

The Turkish army has hit a total of 181 militant targets with air strikes and howitzers since the start of the operation, the defense ministry said on Wednesday.

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Explosions also rocked the Syrian border town of Ras al Ain, according to a reporter for CNN Turk. The sound of warplanes could be heard above and smoke rose from buildings in the town, the CNN reporter said.

Turkish media said several mortar shells had landed on the Turkish side of the border but there were no casualties.

WORSE TURMOIL

World powers fear the Turkish action could open a new chapter in Syria’s war and worsen regional turmoil. Ankara has said it intends to create a “safe zone” in order to return millions of refugees to Syrian soil.

In the build-up to the offensive, Syria had said it was determined to confront any Turkish aggression.

The SDF controls much of the territory that once was held by Islamic State and holds thousands of Islamic State fighters and tens of thousands of their relatives in detention.

It halted operations against Islamic State because of the Turkish offensive, two U.S. officials and a Kurdish source said.

One of the prisons where ISIS detainees are held was struck by a Turkish air strike, the SDF said on Twitter.

The Kurdish-led authority in northern Syria declared a state of “general mobilization” before calling on its people to head toward the border “to fulfill their moral duty and show resistance in these sensitive, historic moments”.

Erdogan’s communications director Fahrettin Altun said Turkey had no ambition in northeastern Syria except to neutralize the threat against Turkish citizens and to liberate the local people from what he called “the yoke of armed thugs”.

Turkey was taking over leadership of the fight against Islamic State in Syria, he said.