US military aircraft conduct their first airstrikes against the Islamic State - the militant jihadi group that controls much of Iraq, as stranded refugees beg for international assistance.

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A Pentagon spokesman announced on Friday that two F/A-18 jets (F/A-18 pictured, above) had dropped 500-pound, laser-guided bombs on Islamic State mobile artillery which was being used against Kurdish forces in Irbil.

Pentagon press secretary Rear Admiral John Kirby said: "The decision to strike was made by the US Central Command commander under authorisation granted him by the commander in chief."

He said Islamic State had been using the artillery to shell Kurdish forces defending Irbil where US personnel are located.

On Friday, a day after President Obama authorised airstrikes on Iraq saying they would protect Christians and Yazidi people, US Secretary of State John Kerry said that "[Islamic State's] campaign of terror against the innocent, including the Christian minority, and its grotesque targeted acts of violence show all the warning signs of genocide."

Humanitarian crisis

Thousands of the minority Yazidi community have been trapped on an Iraqi mountainside by Islamic State fighters. A Turkish official said that trucks carrying food, medicine, blankets and other basic goods would cross into Iraq on Sunday.

Read more: Islamic State fighters kidnap 500 women - reports

'Existential threat'

Irbil, the capital of the Kurdistan region in Iraq, is the latest Islamic State target as it seeks to expand its caliphate state.

Read more: Fear is the key in Islamic State's purge of Iraq

Though at the start of Islamic State's campaign in Iraq it did not engage primarily with Kurdish peshmerga forces, the group has now turned its sights on a city with a population of approximately 1.5 million, mostly Kurdish people.

A senior Kurdish official said the militants now posed an "existential threat" to the Kurds. He also confirmed that Islamic State had taken control of Iraq's biggest dam.

On Friday the British Foreign Office advised all British nationals to leave the Irbil region due to the fighting.

The British Defence Secretary Michael Fallon said although UK was supportive of the US operation, it would not be getting involved in air strikes. The British military would, however, provide humanitarian aid, he said.

Mr Fallon said: "What we have decided today is to assist the United States in the humanitarian operation that started yesterday. We are offering technical assistance in that, in terms of refuelling and surveillance, where we can support that operation."

He added that the UK aid effort would start "over the next couple of days" in a bid to help those who are trapped on the mountain by Islamic State forces in Iraq.

The defency secretary said: "We welcome what the Americans are doing now, in particular to bring humanitarian relief and to prevent any further suffering. But our focus is on assisting that humanitarian mission and on using our military in support of the Americans, in terms of refuelling and surveillance to underpin their mission and to add to it with food drops of our own."

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Political pressure

With efforts to form a unity government in Baghdad still ongoing, Iraqi prime minister Nouri al-Maliki came under further pressure on Friday as the country's top cleric Ali al-Sistani said in his Friday sermon that "clinging to a political post despite the negative consequences on the country is a grave mistake."

The US also chose to underline the need for an inclusive government, with White House spokesman Josh Earnest saying that if an administration was formed that reflected the diversity in Iraq, the US would be prepared to deploy "military force" to help the Iraqis tackle the threat posed by Islamic State, although he said this would not mean combat troops on the ground.

Flights suspended

The Federal Aviation Administration on Friday restricted all US airlines from flying in Iraqi airspace, due to the violence in the region. The decision was followed by an announcement from British Airways that it would be "temporarily suspending our flights over Iraq and will keep the situation under review."

Virgin Atlantic Airways, Air France and Emirates have already stopped flying over Iraq.