The United Nations warned Saturday that Shiite Turkmen residents of an Iraqi town besieged by jihadists since June risk "massacre" if urgent action is not taken to rescue them.

The town of Amerli, in Salaheddin province north of Baghdad, has been entirely cut off from the rest of government-held territory since fighters spearheaded by the Islamic State (IS) swept through much of the rest of the province in a lightning offensive in early June.

"The situation of the people in Amerli is desperate and demands immediate action to prevent the possible massacre of its citizens," UN Iraq envoy Nickolay Mladenov said in a statement.

"I urge the Iraqi government to do all it can to relieve the siege and to ensure that the residents receive lifesaving humanitarian assistance or are evacuated in a dignified manner," he said.

"Iraq's allies and the international community should work with the authorities to prevent a human rights tragedy."

The Shiite faith of the majority of Amerli's 20,000 residents is anathema to the Sunni extremists of IS.

Prime minister designate Haidar al-Abadi promised aid for the town on Saturday, calling for the provision of "all types of military and logistical support for Amerli."

His comments came after Iraq's most powerful Shiite cleric, Grand Ayatollah Ali al-Sistani, who is revered by millions, sought on Friday to bring attention to Amerli's plight.

"Amerli suffers from a tight blockade in place for two months," and has been defended by "heroic men with limited weapons and ammunition and a severe shortage of food items," Sistani said in remarks read on his behalf.

He called for efforts to break "the siege of (the town) and save its people from the dangers of terrorists."

Residents have pleaded for weeks for military intervention and warned that food, medicines and water are in short supply.

Troops are gathering in areas both north and south of the town in readiness for an attempt to break the siege, army Colonel Mustafa al-Bayati said.

And the local official responsible for the area, Adel Shakur al-Bayati, said that food had been airdropped to the town on Friday, while the army carried out air strikes against militants in the area.