KIRKUK, Iraq - Iraq was massing forces Wednesday for an operation to break a two-month militant siege of the Turkmen town of Amerli, amid growing fears for residents short of food and water.

The imminent counter-offensive comes amid reports that US President Barack Obama is weighing a decision to authorise air strikes and aid drops in the area to assist around 12,000 residents trapped in the northern town.

According to a civilian volunteer commander, thousands of Shia militiamen from groups including Asaib Ahl al-Haq and the Badr Organisation are gathering in the Tuz Khurmatu area of Salaheddin province, just north of Amerli, in preparation for a battle to break the siege.

And an army lieutenant general said that security forces were mobilising in the Jabal Hamreen area, south of Amerli, to launch an attack from the southern flank.

Iraqi aircraft have being targeting positions of Islamic State (IS) militants around Amerli, and carried out nine strikes on Tuesday, an officer said.

There is "no possibility of evacuating them so far", and only limited humanitarian assistance is reaching the town, said Eliana Nabaa, the spokeswoman for the UN mission in Iraq.

UN Iraq envoy Nickolay Mladenov has called for an urgent effort to help Amerli, saying residents who have been under siege for more than two months face a "possible massacre" if it is overrun.

Residents say they are suffering from a major shortage of food and water and there is no electricity.

The New York Times reported that Obama is "nearing a decision" on authorising strikes and aid drops in the Amerli area.

The paper added that Obama is also seeking to piece together an international coalition for potential military action in Syria, where the US has begun reconnaissance flights to track Islamic State militants.

The US focus on Syria comes after President Bashar al-Assad's regime said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters.

But American officials said they did not plan to coordinate with Damascus on targeting IS militants in Syria, despite Syrian insistence that any military action on its soil must be discussed in advance.

International concern about IS has been rising after a lightning offensive by the group through parts of Iraq and a string of brutal abuses, including the murder of US journalist James Foley.

A UN-mandated probe charged Wednesday that public executions, amputations, lashings and mock crucifixions have become a regular fixture in militant-controlled areas of Syria.

"In areas of Syria under (Islamic State) control, particularly in the north and northeast of the country, Fridays are regularly marked by executions, amputations and lashings in public squares," the independent Commission of Inquiry on the human rights situation in Syria said.

Militants are also pushing residents, including children, to attend public executions by beheading or a shot to the head, it said.

"Executions in public spaces have become a common spectacle on Fridays," said the report, which also accused Damascus of repeatedly using chemical weapons against civilians.

The US began air raids against IS in Iraq on August 8, in a bid to roll back its advances.

And nine countries have now committed to providing arms to Iraq's Kurdish peshmerga troops, who are fighting IS militants in north and east Iraq.

Pentagon chief Chuck Hagel put the number of countries on board at eight, while Kurdish regional president Massud Barzani said that Iran has provided arms and equipment as well, bringing the total to at least nine.

US Chairman of the Joint Chiefs of Staff General Martin Dempsey has acknowledged that the IS group cannot be defeated "without addressing that part of the organisation that resides in Syria".

Syria's forces meanwhile on Wednesday lost further ground to rebels, who seized control of the Syrian crossing with the Israeli-occupied Golan Heights, a monitoring group said.

The takeover of the Quneitra crossing was led by Al-Qaeda's affiliate Al-Nusra Front joined by a number of rebel groups, the Britain-based Syrian Observatory for Human Rights said.

"Heavy fighting with the Syrian army is continuing in the surrounding area," said Observatory director Rami Abdel Rahman.