ANKARA, Turkey (AP) — After a pre-dawn barrage of heavy artillery and airstrikes, Turkish tanks and special forces pushed into Syria on Wednesday in a U.S.-backed assault aiming to retake a border town from Islamic State militants and contain Kurdish expansion. It marked the NATO member's most significant military involvement so far in the Syria conflict.

The Turkish and Syrian governments said the cross-border incursion on the town on Jarablus was backed by U.S. airstrikes. Hundreds of Syrian opposition fighters also joined the assault. Just hours after the operation began, U.S. Vice President Joe Biden landed in Ankara.

The unprecedented incursion marked a dangerous escalation in the Syrian conflict — and demonstrates the twisted rivalries that run through the war.

The U.S. has long pushed for more aggressive action by Turkey against the Islamic State group. But Turkey's move to thwart Kurdish ambitions puts it on a path toward potential confrontation with Kurdish fighters in Syria who are also supported by the United States and have been the most effective force battling IS in northern Syria.

Turkey has been deeply concerned by the advances of the main U.S.-backed Syrian Kurdish militia, known as the YPG, which after months of taking territory from IS is poised to control nearly the entire Syrian side of the border with Turkey. The YPG is also linked to Kurdish rebels waging an insurgency in southeastern Turkey.

Speaking in Ankara, Biden backed Turkey's demand for limits on Kurdish expansion. Kurdish forces "must move back across the Euphrates River. They cannot, will not, under any circumstance get American support if they do not keep that commitment," he said.

A senior U.S. administration official said U.S. advisers have been working closely with Turkey on plans for the Jarablus operation, providing intelligence and air cover. The official was not authorized to discuss the military operations publicly and spoke on condition of anonymity.

Hours after the start of the operation, rebels backed by the Turkish forces battled their way through several IS-held villages and entered Jarablus. Capt. Abdel-Salam Abdel-Razzak, spokesman for the Nour al-Din al-Zinki rebel group, told The Associated press that the fighters are combing Jarablus for "pockets" of IS militants.

Turkish President Recep Tayyip Erdogan said the military operation aims to prevent threats from "terror" groups, including the Islamic State and the YPG. He said the operation was in response to a string of attacks in Turkey, including an IS suicide bombing at a wedding party near the border which killed 54 people.

A senior official with the YPG's political arm warned Turkey will pay the price. Saleh Muslim, the co-president of the Democratic Union Party or PYD, tweeted that "Turkey is in Syrian Quagmire. Will be defeated as Daesh" will be. He used the Arabic language acronym for IS.

Turkish Foreign Minister Mevlut Cavusoglu accused the Kurds have a secret agenda to establish a state. "If his (Muslim) intention had been to fight Daesh then he wouldn't oppose such an operation... And so the PYD's secret agenda is out in the open."

IS-held Jarablus is a key lynchpin in the Turkish-Kurdish rivalry. The town lies on the western bank of the Euphrates River at the point where it crosses from Turkey into Syria.

The YPG and other Syrian Kurds stand on the east bank of the river, and from there they hold the entire border with Turkey all the way to Iraq. They also hold parts of the border further west, so if they ever took control of Jarablus, they would control almost the entire stretch.

Last month, Kurdish forces and their allies scored a major victory, crossing west of the Euphrates to retake the town on Manbij from Islamic State militants. They now say they will push further west to assault the IS-held town of al-Bab.

Turkey codenamed its cross-border assault "Euphrates Shield," suggesting the aim was to keep the YPG east of the Euphrates River.

Turkish Prime Minister Binali Yildirim said that in Biden's talks in Ankara, the two sides reached agreement that that the Syrian Kurdish forces "should never spread west of the Euphrates and not enter any kind of activity there."

Cavusoglu said Syrian Kurdish forces must cross back to the east side of the Euphrates as soon as possible. "Otherwise, and I say this clearly, we will do what is necessary."

The Syrian government denounced the Turkish military incursion and called for an immediate end to what it described as a "blatant violation" of Syrian sovereignty. It said Turkish tanks and armored vehicles crossed into Syria under the cover of U.S.-led airstrikes.

Turkey has backed rebels against Syrian President Bashar Assad throughout Syria's civil war. It has conducted small, brief special forces operations in the past.

But Wednesday's assault was its first major ground incursion.

The operation began at 4 a.m. (0100 GMT) with intense Turkish artillery fire on Jarablus, followed by Turkish warplanes bombing IS targets in the town. Then up to 20 tanks and a contingent of special forces moved across the border, according to Turkey's private NTV television and several Syrian opposition activists.

Ahmad al-Khatib, a Syrian opposition activist embedded with the rebels, said some 1,500 opposition fighters were involved. He said the fighters come from the U.S.-backed Hamza brigade, as well as rebel groups fighting government forces in Aleppo, such as the Nour el-Din el Zinki brigade, the Levant Front, and Failaq al-Sham.

Fighters from the powerful and ultraconservative Ahrar al-Sham brigade are also present, he said.

Biden's visit comes at a difficult time for ties between the two NATO allies. Turkey is demanding that Washington quickly extradite a U.S.-based cleric blamed for orchestrating last month's failed coup while the United States is asking for evidence against the cleric and that Turkey allow the extradition process to take its course.

But Biden's comments put Washington and Ankara on the same page on limiting the advances by the United States' other main ally in the conflict, the Syrian Kurds.

The capture of the town of Manbij last month from IS heightened Turkey's fears. It was seized by the Kurdish-led group known as the Syria Democratic Forces, or SDF. The U.S. says it has embedded some 300 special forces with the SDF, and British special forces have also been spotted advising the group.

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Issa reported from Beirut. Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.