WASHINGTON (AFP) - US military planes conducted a fourth air drop of food and water for civilians besieged by jihadists on a mountain in northern Iraq, the Pentagon said.

A C-17 and three C-130 cargo aircraft dropped 88 bundles of supplies that will provide "food and water for thousands of Iraqi citizens" trapped on Mount Sinjar, said a statement by US Central Command, which covers the Middle East.

The bundles included 4,394 gallons of drinking water and 22,488 meals ready to eat.

The Pentagon added that the US has delivered "more than 74,000 meals and more than 15,000 gallons of fresh drinking water" to the displaced civilian refugees from the Yazidi religious minority, who have been cornered on the mountain by the so-called Islamic State forces.

The United States on Friday began a campaign of air strikes aimed at halting the advance of the Sunni militants. US President Barack Obama said he was sending warplanes back over the skies of Iraq for the first time since the last US troops withdrew in 2011.

The announcement followed spectacular gains made by the jihadists, who seized the country's largest dam and advanced within striking distance of the Kurdish capital. Obama also justified the return by the risk of an impending genocide against the Yazidi minority.