The Obama administration will cut all U.S. support for its Syrian Kurdish allies, considered the most competent rebel force fighting the Islamic State, if they do not comply with Turkish demands that they withdraw to the east of the Euphrates River, Vice President Biden said here Wednesday.

Biden said the Kurds, who Turkey claims intend to establish a separate state along a border corridor in conjunction with Turkey’s own Kurdish population, “cannot, will not, and under no circumstances will get American support if they do not keep” what he said was a commitment to return to the east.

The primary goal of Biden’s day-long visit here was to convince Turkey that the United States had no role in, and did not condone, a July 15 coup attempt that has sent the country into a whirlwind of conspiracy theories, mass arrests and estrangement from Washington at a key moment in the campaign against the Islamic State.

[How Turkish government regained control after failed coup]

In statements and news conferences following lengthy separate meetings with President Recep Tayyip Erdogan and Prime Minister Binali Yildirim, Biden, the first senior U.S. official to visit Turkey since the coup attempt, said he understood “how some of your countrymen feel the world didn’t respond . . . rapidly enough or with the appropriate amount of solidarity and empathy.”

A handout photo provided by the Turkish Presidential Press Office on Aug. 24, 2016, showsf Turkish President Recep Tayyip Erdogan (R) shaking hands with Vice President Biden. EPA/TURKISH PRESIDENTIAL PRESS OFFICE (Turkish Presidential Press Office/Handout/EPA)

“I personally, [President Obama] personally, and the American people stand in awe at the courage of your people” in standing up to the plotters, Biden said, as Erdogan sat stone-faced beside him.

Obama, Biden said in a news conference with Yildirim, had asked him “to come today in order to remind the world of the paramount importance that we place on the relationship between our two nations, as allies, as partners and friends.”

“Let me be clear, as clear as I possibly can,” Biden said, raising his voice to a near-shout. “I want to ease any speculation, some of which I have heard. . . . The United States of America did not have any foreknowledge of what befell you on the 15th. The United States of America, the people of the United States of America, abhor what happened and under no circumstances would support anything remotely approaching the cowardly act of the treasonous members of your military.

“We did not have prior knowledge, we did not support, we immediately condemned, and we continue as we did before the coup to stand shoulder to shoulder not only with the government of Turkey but with the people of Turkey. The people of Turkey have no greater friend . . . than the United States of America.”

Biden said the administration is acting expeditiously to process Turkey’s request to extradite cleric Fethullah Gulen, a U.S. resident who Ankara says was the mastermind of the coup attempt. He made no mention of U.S. concerns, stated by Obama and others, that Turkey is diluting the rule of law and its democratic norms with the arrests and detentions of tens of thousands of alleged supporters of the plot.

A Justice Department team arrived here Tuesday to clarify some of the information contained in four separate Turkish requests, all of which refer to alleged criminal activity by Gulen predating the coup attempt. But officials have said that the required steps — including an Obama administration determination of whether the evidence crosses the legal threshold for extradition, followed by an extended court process — may take years.

[Turkey says this frail cleric is a coup mastermind ]

In response to a question, Biden said he believed the Turks when they said they would uphold democracy and human rights. Citing what he called the “confusion” in the United States after the attacks of Sept. 11, 2001, he said, “Let’s give this some time.”

Neither Erdogan nor Yildirim gave much public ground in response to Biden’s comments. Asked directly whether his government, which has issued thinly veiled allegations of U.S. complicity with the coup plotters, now believed in American innocence, Yildirim said that speedy extradition of Gulen would help the Turkish people get over their suspicions.

Erdogan suggested that Gulen be immediately arrested while extradition was being processed. Biden responded that, just as extradition is a matter for the courts and not the executive branch, a court would have to decide whether there is probable cause to detain Gulen.

But it was Biden’s remarks about the Kurds that held the most potential for a substantive U.S. policy change. The comments are likely to provoke a negative response from U.S. critics who already think that Obama’s approach to Turkey, a country long accused of sympathizing with Islamist extremists in Syria, has been too conciliatory.

The Kurdish force has been instrumental, along with airstrikes, in what the administration has claimed as a series of rebel victories against the militants over the past year in Syria, including the retaking this month of the northern city of Manbij and other key towns west of the Euphrates River.

Turkey, Yildirim said, considers the Kurdish People’s Protection Units, or YPG, a terrorist organization. It says, and the administration has not disputed, that the United States promised it would not allow YPG forces to expand the border area they now control from the Euphrates east to the Iraqi border.

Biden’s arrival here came only hours after Turkish forces, with U.S. air support, crossed into Syria to eject the Islamic State from the town of Jarabulus, the militants’ last remaining stronghold along the Syrian-Turkish border. Yildirim said Turkey’s motivation was to vanquish the Islamic State and to ensure that Kurdish fighters did not move into position there from nearby Manbij.

A senior administration official traveling with Biden acknowledged that the Kurds had begun to head north from outside Manbij over the weekend and that they were met with cross-border Turkish shelling. The official said the United States had informed Turkey well before the Manbij operation that the Kurds were under strict instructions to return to the east side of the Euphrates after the city was captured.

In a terse but unambiguous response to a question about the Kurdish moves, Biden ruled out a Kurdish border enclave. There will be “no [Kurdish] corridor,” he said. “Period. No separate entity on the border. A united Syria.”

“We have made it absolutely clear to . . . the YPG that participated” in the taking of Manbij and other towns “that they must move back across the river,” he said. “They cannot, will not, and under no circumstances will get American support if they do not keep that commitment. Period.”

Read more:

Today’s coverage from Post correspondents around the world