The United States has no plans to expand its air campaign in Iraq beyond protecting American personnel in the city of Arbil and besieged Yazidi refugees, the Pentagon said Monday.

"There are no plans to expand the current air campaign beyond the current self defense activities," Lieutenant General William Mayville told reporters at the Pentagon.

Last week, US warplanes launched strikes to beat back extremist fighters from the so-called Islamic State who had threatened to massacre the Yazidi religious minority and attack Arbil.

Arbil, the capital of Iraq's autonomous Kurdish region, hosts a US consulate and other facilities, and President Barack Obama said he had ordered air strikes to protect American lives.

Meanwhile, US planes joined Iraqi forces in attempting to bring aid to thousands of Yazidis trapped on an exposed mountain and under attack by the IS militants who had driven them from their homes.

The sight of US jets and drones in action over Iraq again, less than three years after American forces withdrew from the country, has raised fears that Washington is once again being dragged into war.

But Mayville insisted that there had been no mission creep.

"Our principal task to date, and what we are doing right now, is to protect US facilities and the citizens -- American citizens -- at those facilities," he said

In addition, he said, the mission was "to protect US aircraft that are supporting humanitarian assistance around Mount Sinjar and to target those ISIL positions that are laying siege to Mount Sinjar."

There have been reports that US or allied forces might attempt to open a humanitarian corridor to Sinjar to rescue refugees en masse, but Mayville stressed that the situation was "complicated."

"We are right now gripped by the immediacy of the crisis, and our focus right now is to provide immediate relief to those that are suffering," he said.

"We are looking at the effect that we're having on those fixed sites, those ISIL sites laying siege, and we are trying to reduce that threat. And for the near term, that's going to be our focus."

US and Iraqi aircraft have managed to drop some aid to Yazidi refugees, and others have been rescued by Iraqi helicopters or Kurdish troops -- but many more are thought to be trapped.

Mayville said US aircraft had flown "between 50 and 60 sorties per day" since the start of the operation on Thursday last week, but did not say how many were aid drops and how many deadly strikes.

Britain aborts second Iraq aid drop over safety fears
London (AFP) Aug 11, 2014 - Britain was forced to abort a second airdrop of humanitarian aid to refugees in northern Iraq on Monday because of fears of injuring people on the ground, a military spokesman said.

A fresh attempt will be made within 24 hours to deliver desperately needed food and water to Yazidis stranded on Mount Sinjar after they fled jihadist Islamic State (IS) fighters.

Foreign Secretary Philip Hammond, who chaired a meeting of the government's emergency Cobra committee to discuss the crisis, said the situation was "challenging" and warned of a "potential humanitarian disaster on a huge scale".

But he rebuffed calls from some lawmakers to recall parliament to authorise airstrikes on IS fighters which the US is already carrying out.

Hammond said: "We are providing humanitarian assistance. This is not simple -- getting it in is very challenging, getting people off that mountain is even more challenging."

He said the meeting had discussed "obtaining better situational awareness of what's going on on the mountain, both to facilitate the air drops and to start planning how we are going to get people out".

A Royal Air Force (RAF) spokesman said the air crew aborted the drop because they had been afraid of hurting people at the crowded site.

"With a number of people at the drop sites this morning, the crew made the responsible decision not to carry out the air drop to ensure that the lives of those in the area would not be put at risk," he said.

"We plan to deliver the next drop as soon as possible."

Hammond said the plight of the Yazidis was a "potential humanitarian disaster on a huge scale" and it was up to Britain and other countries to help.

Two transport planes left Britain on Saturday and the first drop was made that night, including 1,200 water containers providing 6,000 litres of water, and 240 solar lanterns that double as phone chargers.

Britain has also offered surveillance and refuelling support for US aid flights, but the government says it is not planning to follow the United States' lead and launch airstrikes.

"The American airstrikes are concentrated on protecting Arbil where there is a significant concentration of American personnel," Hammond said.

"We don't envisage a combat role at the present time, we are talking at the moment about a humanitarian operation."

The United States also said on Monday it has begun urgently shipping arms and ammunition to the Iraqi Kurdish forces fighting IS forces.

"We do think it's important that the Iraqi forces, including the Kurdish forces, are able to respond to IS and to tackle this crisis in the country," a spokeswoman for Prime Minister David Cameron's Downing Street office said.

"We will look at what options there are that might enable them to do that. But there have not been substantive discussions on that yet and there are certainly no decisions."