The Iraqi government says it is troubled by reports that Islamic State militants are selling oil from wells they have captured in northern Iraq.

Baghdad has warned buying such supplies could help the group fund its operations, as the US revealed it has carried out nearly a dozen drone airstrikes since Tuesday as it tries to push back the extremists.

Military planners in the US are also considering sending more American forces to Iraq, mainly to provide extra security around Baghdad.

Foreign Minister Hoshyar Zebari has urged the world to back Iraq against IS, which was previously known as ISIS.

Mr Zebari described the fighters as a threat to the world, and not just the minority ethnic groups IS has killed in Iraq.

Thousands have been killed as IS seized swathes of territory in an offensive that has also forced 1.5 million people from their homes.

IS has released a video that appears to show a masked man - who speaks with an English accent - beheading US journalist James Foley.

The footage came a day after militants threatened to attack US targets in another video clip.

US airstrikes have helped Kurdish and government forces push IS fighters back from recently-conquered areas in the north of the country, including Mosul Dam.

Among the latest strikes, an apparent jihadist meeting at a school near the strategically important dam was targeted, Kurdish officers said, although Washington has not confirmed the attack.

Germany and Italy have said they are ready to send arms to bolster the military capabilities of forces fighting IS, with Berlin denouncing the militants as "barbaric".

France began shipping weapons to Kurdish forces last week.

As well as military support, aid has been delivered to those caught up in the fighting by Germany, France and Britain.

The United Nations' refugee agency, UNHCR, said on Wednesday that a humanitarian airlift to northern Iraq had begun.

The agency said 100 tonnes of emergency supplies, including plastic sheeting for shelter, kitchen sets and jerry cans, were on a cargo plane that touched down in the Kurdish regional capital, Irbil.

It is the start of a 10-day operation to provide tents and other aid to half a million displaced people.

President Barack Obama ordered US warplanes to begin striking IS fighters earlier this month, arguing they threatened US personnel in Irbil and risked carrying out a genocide against religious minority groups.

But while he has insisted the scope of the strikes would remain limited, Iraqi officials have argued that only foreign intervention can overturn IS's gains.

An offensive launched on Tuesday to secure Tikrit was presented as a major push to liberate the city, but this appears to have stalled.

The hometown of former dictator Saddam Hussein was hit by shellfire on Wednesday while Iraqi security forces remained positioned outside of the city, police and witnesses said.