Next month's referendum on Iraqi Kurdish independence violates Iraq's constitution and will further destabilise the region, a Turkish government spokesman said on Tuesday.

Iraq's Kurds have said they will go ahead with the referendum on independence on 25 September despite concerns from Iraq's neighbours who have Kurdish minorities within their borders, and a U.S. request to postpone it.

"The referendum would contribute to instability in the region," Turkey's Deputy Prime Minister and government spokesman Bekir Bozdag told a news conference after a cabinet meeting in Ankara, adding the decision to go ahead with the vote "violates the constitution of Iraq".

Turkey's outlawed Kurdistan Workers Party (PKK) has waged a three-decade insurgency against the Turkish state.

In Syria, where President Bashar al-Assad's government has lost control of large parts of the country, Kurdish YPG fighters hold territory along the border with Turkey and the Kurdish-led administration plans local elections next month - a move Damascus has rejected as a "joke".

The U.S. State Department has said it is concerned that the referendum in northern Iraq will distract from "more urgent priorities" such as the defeat of Islamic State militants.

Turkish Energy Minister Berat Albayrak said last week the referendum would harm energy cooperation with northern Iraq's Kurdish regional authority, which pumps hundreds of thousands of barrels of oil a day to Turkey's Ceyhan export terminal.