The UN has declared its highest level of emergency in Iraq as a humanitarian crisis follows the rapid advance by Islamic State militants in the north.

Kurdish officials said the situation in Dohuk city, with 150,000 refugees, was now critical.

But the US said a rescue mission to aid thousands who fled to Mt Sinjar was unlikely as it had found fewer people and better conditions than expected

The UN estimates that 1.2 million Iraqis have been internally displaced.

The three other countries that have the same emergency status are Syria, South Sudan and the Central African Republic.

Speeding up response The declaration by the UN of a "Level 3 Emergency" would "facilitate mobilisation of additional resources in goods, funds and assets to ensure a more effective response to the humanitarian needs of populations affected by forced displacements", said UN special representative Nickolay Mladenov.

The situation of displaced people on Mount Sinjar remained critical, he said. The UN had estimated that tens of thousands of people, most from the Christian and Yazidi religious minorities, were besieged on the mountain after being forced to flee their homes. US forces conducted a brief mission to Mt Sinjar to assess the situation after days of international concern. It found fewer people than expected and that those remaining were also in a better condition than feared - partly because thousands had left the mountain each night over the past days. Displaced Iraqi Yazidis, who fled a jihadist onslaught on Sinjar, stand inside a tent after they took refuge at the Bajid Kandala camp in Dohuk province 13/08/2014 Some Yazidis have taken refuge at a camp in Dohuk province Temporary shelter in the Christian quarter of Ainkawa, Irbil 13/08/2014 Other religious minorities have also had to flee their homes, like these Christians in Irbil An F/A-18F Super Hornet of Strike Fighter Squadron (VFA-213) (R) and an F/A-18C Hornet of Strike Fighter Squadron (VFA-15) prepare for take off onboard the aircraft carrier USS George H.W. Bush in the Gulf, 13 August 2014 The US has been carrying out air strikes on IS targets It said it would continue to provide humanitarian assistance there. Planes have been dropping aid supplies on Mount Sinjar for several nights. The US said it conducted its seventh air-drop of food and water on Wednesday, and had delivered more than 114,000 meals and 35,000 gallons (160,000 litres) of drinking water to those trapped.

Kurdish officials in Dohuk told the BBC's Frank Gardner that there are 150,000 refugees there, overwhelming the local population who are trying to feed them. They need tents, food and water. The US has also continued to conduct air strikes on IS targets in Iraq's north.

In another development, the governor of Anbar province - large parts of which have been under militant control for months - has asked for US air support and other assistance, Reuter reports.

The agency said Ahmed Khalaf al-Dulaimi feared opponents of Islamic State in the vast and predominantly Sunni province may not have the stamina for a long fight.

France says it will arm Iraq's Kurds, who are already getting US military aid to fight IS. Meanwhile, the UK has pledged to join a rescue mission for displaced civilians.

Jump media playerMedia player helpOut of media player. Press enter to return or tab to continue. James Robbins reports on the humanitarian crisis

The rapid advance across Iraq by Islamic State fighters has thrown the country into chaos. On 29 June, IS said it had created a caliphate, or Islamic state, stretching from Aleppo in Syria to the province of Diyala in Iraq.

Iraq map