Tens of thousands of civilians remain trapped by militants on a mountain in northern Iraq and need "life-saving assistance", the United Nations warns.

Members of the Yazidi sect fled there 10 days ago after fighters from Islamic State (IS) seized the town of Sinjar.

US air strikes continued today, with a drone targeting an IS mortar near Kurdish troops, the Pentagon said.

IS fighters have seized large swathes of northern Iraq and Syria in recent months, forcing tens of thousands of people from religious minorities to flee their homes.

An estimated 20,000 to 30,000 people remain trapped on Mount Sinjar without food, water or shelter said the UN's Adrian Edwards in a statement.

There are now an estimated 1.2 million internally displaced Iraqis, he added.

Some Yazidis had managed to escape from the north side of the mountain in the last 72 hours and cross the River Tigris into Syria, where they were receiving help.

But a Yazidi relief worker, who was on board an Iraqi Air Force helicopter evacuating the trapped refugees, described the situation as "a genocide" and said that he had seen what looked like "hundreds" of dead bodies on the ground.

"You can imagine what it's like when you land amongst 5,000 people and can only take 10 or 20, and everybody tries to get on the helicopter," Mirza Dinnay said.

The US, Britain and France have been delivering humanitarian aid to the Yazidis.

Over the last five nights, US and UK air crews have air-dropped more than 310 bundles of food, water and medical supplies, and delivered almost 72,740 litres of water and 75,000 meals, the US government says.

The US has also reportedly begun supplying weapons to the Kurdish forces, known as Peshmergas, who have been fighting IS in the north.