The UN has called for action to prevent what it says may be a possible massacre in the northern Iraqi town of Amerli.
Special representative Nickolay Mladenov says he is "seriously alarmed" by reports regarding the conditions in which the town's residents live.
Amerli, under siege by Islamic State (IS) for two months, has no electricity or drinking water, and is running out of food and medical supplies.
IS has seized large swathes of Iraq and Syria in recent months.
Since 8 August, the US has carried out 94 air strikes to support Iraqi and Kurdish troops tackling the insurgents.
'Immediate action' needed
The majority of Amerli's residents are Turkmen Shia, seen as apostates by IS.
The town's inhabitants say they have had to organise their own resistance to the militants and no foreign aid has reached the town since the siege began.
"The situation of the people in Amerli is desperate and demands immediate action to prevent the possible massacre of its citizens," Mr 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 life-saving humanitarian assistance or are evacuated in a dignified manner."
Mr Mladenov's deputy, Gyorgy Busztin, told the BBC that the UN had no contact with IS representatives.
"We are not talking to terrorists and this is a matter of principle," he said.
"There is no way anybody can have any positive effect on these people. We have contacts with moderate Sunnis connected to the... areas which [IS] has overrun."
On Friday, the most influential Shia cleric in Iraq, Grand Ayatollah Ali al-Sistani, expressed concern over the plight of Amerli's inhabitants.
The rise of IS has sparked widespread violence.
- On Saturday, a suicide bomber blew up a car in central Baghdad, killing at least nine people and injuring several others.
- In the Kurdish-held city of Kirkuk, at least 18 people including security personnel were killed by simultaneous car bombs.
- Another bomb exploded in the Kurdish regional capital Irbil, a rare occurrence in a region that has seen far less violence than elsewhere in Iraq.
- An attack by suspected Shia militiamen on a Sunni mosque in Iraq's Diyala province killed at least 68 people on Friday.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- By early 2014, it controlled Falluja in western Iraq
- Has since captured broad swathes of Iraq, seizing the northern city of Mosul in June
- Fighting has displaced at least 1.2 million Iraqis
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- In July alone, IS expanded dramatically, recruiting some 6,300 new fighters largely in Raqqa, an activist monitoring group said
On Thursday, US Defence Secretary Chuck Hagel described IS as an imminent threat to the US.
Gen Martin Dempsey, the chairman of the US Joint Chiefs of Staff, said IS was "an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated".
He said that IS fighters' bases in Syria also had to be attacked.
The Shia-dominated Iraqi government has been trying to secure backing from Sunni groups in its battle against IS jihadists.
Prime Minister designate Haider al-Abadi, a moderate Shia, is trying to form a more inclusive government - following international criticism of outgoing PM Nouri Maliki, who was widely seen as a divisive figure.
The IS campaign has displaced an estimated 1.2 million people in Iraq, many of them minority Christians and Yazidis.
Refugees say the hardline Islamists have demanded that Christians and Yazidis convert to Islam, threatening them with death if they refuse.