By Tamara Qiblawi, Nima Elbagir and Ghazi Balkiz, CNN

(CNN) -- Iraqi Kurds have started casting their ballots in a controversial independence referendum Monday as tensions between Iraq's largest ethnic minority and the Iraqi government in Baghdad intensifies.

Voting got underway at 8 a.m. local time (1 a.m. ET) and will end at 6 p.m. local time (10 a.m. ET). The first results should be known within 72 hours.

The Kurdistan Regional Government (KRG) which administers a semi-autonomous region in northern Iraq says the referendum will give it a mandate to achieve independence from Iraq.

Ballot boxes are spread throughout Iraq's Kurdish regional areas, in addition to the city of Kirkuk which has been for years now a flashpoint claimed by both the KRG and the Iraqi government.

On Sunday, Iran closed its air space to the autonomous Kurdistan region after issuing several condemnations against the vote.

Both Iran and Turkey have sizable Kurdish minorities and fear a vote for independence in Iraq might galvanize movements in their countries.

As polls opened Monday, the Turkish foreign ministry issued a statement calling the vote "illegitimate and null and void."

The statement called on regional states to "refrain from recognizing the consequences of this illegitimate endeavor." It also warned that it would take legal measures to prevent "radical elements and terrorists" from attempting to exploit the situation to target its national security.

The United States, United Kingdom and the United Nations have also warned the KRG against holding the referendum, citing fears that it could detract from the campaign against ISIS.

'Centuries in the making'

On Sunday, buildings in the Kurdish-controlled city of Irbil were decked out in Kurdish flags. Elsewhere, in areas claimed by both the Kurds and the Iraqi government, locals told CNN that clashes have been happening almost every night in the run up to the poll.

Already one person has died in skirmishes.

Supporters of the referendum say the vote has been "centuries in the making."

Kirkuk council member Dilshad Perot Aziz told CNN, "the same reasons that gave rise to ISIS are the reasons that led to the referendum. The corruption, the disenfranchisement, the impunity. All of this wearing on the Iraqi people."

Pro-independence Kurds say that the government in Baghdad has reneged on promises to treat Kurds and Arabs equally.

Others in Kirkuk were concerned that an independent Kurdistan would turn non-Kurds into second-class citizens.

"The policies of the Kurds in Kirkuk is Saddam Hussein's policy," said Ali Mehdi of the Iraqi Turkmen Front referring to the late Iraqi dictator who was routed from power after the US-led invasion in 2003.

"Kirkuk is very tense at the moment and the Kurds are pushing the downtrodden non-Kurdish elements to participate in the referendum," Mehdi said.

Those sentiments were echoed by Arab tribal leader Sheikh Burhan Mezher who showed CNN anonymous Facebook messages with threats to harm his children.

He said he didn't believe the threats were coming from officials but that tensions were running high and "more could be done."

On the road to Mosul, a Kurdish Peshmerga checkpoint erected seven months ago to serve as a bulwark against ISIS has turned into a quasi-border post. It stands at the perimeter of the would-be Kurdish state.

Lieutenant Colonel Mohammed Youssef who supervises the checkpoint told CNN the risk of violence means little to Kurdish people.

"We've been like a thirsty man desperate for water, that's how we've longed for freedom. We've longed for our own state, for our country," Youssef said.

"We paid for this in blood and we're prepared to sacrifice even more."