Iraqi Kurds voted in an independence referendum Monday in defiance of Baghdad which has warned of "measures" to defend Iraq's unity and threatened to deprive their region of lifeline oil revenues.

The non-binding vote on the Kurds' long-cherished dream of independence, initiated by veteran leader Massud Barzani, has angered not only the federal government but also neighbouring Turkey and Iran, concerned it could stoke separatist aspirations among their own sizeable Kurdish minorities.

The United States and other Western nations have also raised concerns, saying the vote could hamper the fight against the Islamic State jihadist group in which cooperation between Baghdad and the Kurds has been key.

Initial results are expected to be announced 24 hours after polls close. An overwhelming "Yes" outcome is expected, but Kurdish officials have said there are no plans for an immediate declaration of independence.

Kurdish flags were festooned in all the streets, on cars and outside homes across Iraqi Kurdistan.

Voters headed to the polls early Monday, many men dressed in traditional Kurdish dress of brown shirt and billowing trousers for the occasion.

Young girls wore caps emblazoned with the Kurdish colours of red, white, green and yellow, and regional flags around their necks and shoulders.

"I came very early to be the first to vote for a Kurdish state," Diyar Abubakr, 33, said outside a polling station in regional capital Arbil.

"It's a day of celebration today. That's why I've put on our traditional outfit, which I bought for the occasion," he said.

- Cow slaughtered -

One voter even brought a cow to slaughter before the start of the referendum.

"I brought this cow as today the state is born and it's tradition to slaughter a cow for a birth," Dalgash Abdallah, 27, said.

Barzani, smiling and wearing a traditional outfit, cast his vote early in the morning.

Polling stations are scattered across the three northern provinces of autonomous Iraqi Kurdistan -- Arbil, Sulaimaniyah and Dohuk -- as well as in disputed bordering zones such as the oil-rich province of Kirkuk.

In Sulaimaniyah, second city of the autonomous region, 40-year-old Diyar Omar came to cast his vote also wearing traditional clothes.

"We will seize our independence through the polls. I'm delighted to be the first voter," he said as he arrived at a polling station in the city.

"I'm so happy I could take part in this independence vote during my lifetime," he added.

A total of 12,072 polling stations are open for more than 5.3 million registered voters.

In disputed Kirkuk, mosque loudspeakers blared out calls for residents to take part in the vote.

Participants, some wearing the Kurdish flag round their necks, showed off their ink-stained fingers after casting their vote.

"If I have 20 fingers, I would have voted 20 times for my state," Ibtissam Mohammed, 45, said.

As Barzani pledged on Sunday to go ahead with the vote after having resisted a barrage of international pressure to call it off, Iraq's leader pledged to take all the "necessary measures" to protect the country's unity and his government targeted the Kurds' oil sales.

- 'Necessary measures' -

Prime Minister Haider al-Abadi said the Kurds' unilateral decision to stage a referendum affected both Iraqi and regional security, and was "unconstitutional and against civil peace".

"We will take the necessary measures to preserve the unity of the country," he said, without elaborating.

Hours later, the Iraqi government at a meeting of its security cabinet called on all countries "to deal only with it on matters of oil and borders".

The Iraqi Kurds export an average 600,000 barrels per day (bpd) through a pipeline running through Turkey to Ceyhan on the Mediterranean.

A Turkish minister on Monday denied media reports that a land border gate with northern Iraq was closed, but said tight controls were in place.

Tehran on Sunday said it had blocked all flights to and from Iraqi Kurdistan at Baghdad's request.

But Barzani said the Kurds' "partnership with Baghdad" since the ouster of dictator Saddam Hussein in a 2003 US-led invasion had failed and urged his people to vote.

"We expect reactions from one side or another but we are convinced that whatever the risk and the price, it's better than waiting for a dark outcome," he said.

"The referendum is not for defining borders or imposing a fait accompli. We want a dialogue with Baghdad to resolve the problems, and the dialogue can last one or two years," Barzani said of zones such as Kirkuk.