Iraqi security forces, Shia militiamen and Kurdish fighters have launched a major operation to attempt to break a more than two-month long jihadist siege of a Shia Turkmen-majority town, officials said.

The operation has been in the works for days, with Iraqi aircraft carrying out strikes and forces massing for the drive toward Amerli, which has been besieged since militants led by the Islamic State jihadist group launched a major offensive in June.

Residents face major shortages of food and water, and are in danger both because of their Shia faith, which jihadists consider heresy, and their resistance to the militants, which has drawn harsh retribution elsewhere.

Army Staff Lieutenant General Abdulamir al-Zaidi said the operation to free Amerli from the jihadists has been launched with support from Iraqi aircraft, vowing that "we will be victorious over them".

Karim al-Nuri, spokesman for the Badr Organisation militia, said thousands of its fighters were taking part alongside civilian volunteers and security forces.

Forces from two other Shia militias - Asaib Ahl al-Haq and powerful Shiite cleric Moqtada al-Sadr's Saraya al-Salam forces - had also been gathering north of Amerli for the attack.

And Karim Mulla Shakur, a Kurdish political party official, said that Kurdish Peshmerga fighters were also involved.

Officials have said that the United States is weighing both aid drops and air strikes to help the town.

"It could be a humanitarian operation. It could be a military operation. It could be both," a US defence official said, on condition of anonymity.

There is "no possibility of evacuating them so far", Eliana Nabaa, spokeswoman for the UN mission in Iraq, has said of Amerli residents.

And UN Iraq envoy Nickolay Mladenov has called for an urgent effort to help Amerli, saying residents face a "possible massacre" if the town is overrun.

Number of Yazidi seeking refuge in Turkey rises

Meanwhile, the number of refugees from Iraq's Yazidi religious minority who have fled to Turkey to escape the advance of jihadists has risen to 16,000, and may rise further, a top provincial official was quoted as saying.

The number marks a sharp rise from the figures of several thousand previously given and shows Turkey is dealing with another major influx of refugees as it already provides sanctuary for some 1.2 million fleeing the Syria conflict.

The Yazidi refugees have fled to the southeastern Turkish province of Sirnak, which borders Iraq,  to escape the murderous advance of IS jihadists who specifically target their community.

Some 2,000 Yazidi refugees have entered Turkey through legal border crossings and another 14,000 through other routes, Sirnak province governor Hasan Ipek was quoted as saying in a statement by his office.

"They are being provided with all kinds of health services and three meals a day," he said, admitting that the sanitary conditions in the schools and homes were they were staying were "unfortunately" not always up to scratch.

Turkey is already setting up a refugee camp for Yazidis in the town of Zakho inside northern Iraq and Mr Ipek said it had to function well, "otherwise we are faced with the prospect of 40,000 refugees entering Turkey."

NTV private television also reports that a tented camp was being set up in Turkey's Diyarbakir province to the north of Sirnak for 3,000 Yazidi refugees to help accommodate the new influx.

The Yazidis are on the run from jihadists who scorn them as "devil worshippers", a term the Yazidis themselves angrily reject.

Turkey's new President and former prime minister Recep Tayyip Erdogan has said that Turkey is the "only country to have opened its doors" to the Yazidis.

Turkey's hosting of the Syrian refugees in line with an open door policy spearheaded by Mr Erdogan has already become a source of friction with local residents.