The United States ramped up pressure Wednesday on Iraq's Kurds to abandon a planned referendum on independence, threatening to withdraw international support for negotiations with Baghdad if the vote isn't scrapped.

In a forceful warning, the Trump administration said the costs of holding the Sept. 25 vote would be high "for all Iraqis, including Kurds." State Department spokeswoman Heather Nauert said the U.S. was urging the Kurds to "accept the alternative" - talks between the northern Kurdish region and Iraq's central government that the U.S. and United Nations would facilitate.

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"If this referendum is conducted, it is highly unlikely that there will be negotiations with Baghdad, and the above international offer of support for negotiations will be foreclosed," Nauert said.

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The Iraqi Kurds intend to hold the vote in three governorates that form their self-ruled region, and also in disputed areas controlled by Kurdish forces but claimed by Iraq's government. That includes Kirkuk, an oil-rich but ethnically mixed province that the Kurds helped liberate from the Islamic State group.

The U.S. has previously voiced opposition to the vote, out of concern an expected "yes" vote would fracture Iraq and destabilize the broader region. But Wednesday's warning was the strongest to date from the U.S., reflecting growing concerns that the vote could imperil hard-fought progress in the country where the U.S. has been fighting since 2003.

Naeurt said the planned referendum has already impeded the U.S.-led campaign to defeat the Islamic State group in its remaining strongholds in Iraq. She said IS and other extremist groups would exploit the tensions that would result from the referendum.

"All of Iraq's neighbors, and virtually the entire international community, also oppose this referendum," Nauert said. One key U.S. ally - Israel - supports Kurdish independence in Iraq.

Iraq's top court has temporarily suspended the vote, and the country's parliament has also voted to reject it. Iraqi Prime Minister Haider al-Abadi has even said he's prepared to intervene militarily if the vote leads to violence.

But Kurdish officials have continued to say the vote will be held nonetheless.

The Kurds are an ethnic group with populations in Iraq, Syria, Iran and Turkey. In Iraq, the Kurds have long aspired to statehood, and were harshly oppressed under Saddam Hussein.