Turkey launched a military operation against Kurdish fighters 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.

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.

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

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

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 militants, ISIL) 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."

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.

UN Secretary-General Antonio Guterres said any military operation must fully respect the U.N. Charter and international humanitarian law. Spokesman Farhan Haq said: "Civilians and civilian infrastructure should be protected. The secretary-general believes that there's no military solution to the Syrian conflict."

European Commission President Jean-Claude Juncker urged Turkey to show restraint and stop its military operation. "If the plan involves the creation of a so-called safe zone, don't expect the EU to pay for any of it," he said.

German Foreign Minister Heiko Maas said Turkey's operation would lead to further detribalization of the region and could strengthen ISIL. He urged Turkey to end the operation.

France's European affairs minister Amelie de Montchalin said France and Britain would call a UN Security Council meeting over the Turkish offensive. France, Germany and Britain are finalizing a joint statement condemning the advance.

Egypt's foreign ministry condemned the offensive as "a blatant and unacceptable attack on the sovereignty of a brotherly Arab state," and called for an emergency meeting of the League of Arab States.