Washington put to one side its longstanding sympathy for its allies in Iraqi Kurdistan on Friday and sternly urged the region to call off its independence referendum.

Earlier, Iraqi Kurdish lawmakers had voted to approve the September 25 vote that was set in motion by regional president Massud Barzani, a Washington ally who has publicly kept open the option of postponing it.

Washington has long supported Kurdish autonomy and has relied on the region's forces in the war against the Islamic State group, but it fears that now is not the time for the people to seize their freedom.

US officials fear the vote, while not legally binding, will hurt Iraqi Prime Minister Haider al-Abadi's re-election chances; complicate ties with Turkey; and disrupt the war against IS.

"The United States has repeatedly emphasized to the leaders of the Kurdistan Regional Government that the referendum is distracting from efforts to defeat ISIS and stabilize the liberated areas," President Donald Trump's White House said, in a statement.

"Holding the referendum in disputed areas is particularly provocative and destabilizing," it warned. While Baghdad recognizes Kurdistan's autonomy, the precise boundary between the region and the rest of Iraq is unclear.

Washington has repeatedly offered to help negotiate a long-term settlement between Arbil and Baghdad, but regional leaders -- including Barzani -- have been increasingly frustrated that warm words have not led to a precise diplomatic timetable.

This week, top US envoy Brett McGurk was again in Arbil and attempted to persuade the Kurdish leader to call off the highly-charged popular vote in exchange for a new diplomatic initiative.

Under this plan, a well-placed source told AFP, the international community will oversee negotiations on revenue sharing in Iraq's oil budget and payment for Kurdish militia fighters.

Borders and military forces would remain in their current positions, and Baghdad would authorize Kurdistan to continue exporting the oil that it currently ships through Turkey in breach of the federal constitution.

Finally, Kurdish parties would take part in the Iraqi government and the 2018 elections.

Analysts, however, told AFP that this would not be enough at this stage to convince Barzani to hold off on an independence vote in which he has invested much of his domestic political capital.

"They were very unlikely to accept a deal unless the deal had some kind of iron-clad specificity and international guarantee," said Michael Knights, an Iraq expert at the Washington Institute for Near East Policy.

"The leaders of the US, Britain and the United Nations would have had to commit to the date by which Kurdistan and Iraq would have negotiated Kurdish sovereignty -- or commit to supporting a Kurdish unilateral declaration of independence."

Accordingly, and in the face of bitter opposition from Baghdad, 65 out of 68 lawmakers present voted in favor of the September 25 poll as opposition members boycotted the parliament's first session in two years.

After the show of hands, lawmakers stood to sing the Kurdish anthem while others raised flags to the sound of applause.

The vote was to give a legal framework to the referendum that has also stirred protests from neighboring states, especially Turkey.

- 'Very bad thing' -

The session was the regional parliament's first in two years, and Barzani's mandate as president of the autonomous Kurdish region in northern Iraq officially expired in 2015.

The Kurdish leadership, made up of Barzani's Kurdistan Democratic Party (KDP) and the Patriotic Union of Kurdistan (PUK) of Iraq's former president Jalal Talabani, have maintained that the three-year-old battle to drive back IS has made it impossible to hold fresh elections.

Two opposition parties -- the independent Goran, which has 24 seats in the 111-seat parliament, and Jamaa Islamiya, which is close to Iran and holds six seats -- had said they would boycott the session.

Friday's session in Arbil followed two anti-referendum votes which passed earlier this week in the national parliament in Baghdad, both of which were boycotted by Kurdish legislators.

Analysts say the referendum plan, which has stirred Arab-Kurdish ethnic tensions, could mark the end of an era of cooperation during which Baghdad and Arbil battled IS together after it seized swathes of northern and western Iraq in the summer of 2014.

Turkey and Iran fear the referendum could stoke separatist aspirations among their own sizeable Kurdish minorities.

- Divided city -

Turkish President Recep Tayyip Erdogan has warned the vote could prove "a very, very bad thing" for the Iraqi Kurds, whose economy is heavily dependent on oil exports via a pipeline running through Turkey.

Turkey's National Security Council will meet on September 22 to decide its official position.

On Thursday, the Baghdad parliament fired the governor of the northern province of Kirkuk, Najm Eddine Karim, over his provincial council's decision to take part in the non-binding Kurdish referendum.

The oil-rich province is disputed by Baghdad and Arbil and home to diverse communities including Arabs and Turkmens who oppose the vote.

Iraqi Kurdistan, whose people were brutally repressed under Saddam Hussein, won autonomy following the dictator's ouster in a US-led invasion, under a 2005 constitution which set up a federal republic in Iraq.