Islamic State militants have taken 550 families from villages around Mosul and are holding them close to Islamic State locations in the Iraqi city, probably as human shields, a spokeswoman for the UN human rights office said.

UN spokeswoman Ravina Shamdasani, citing "corroborated information" from the area, said the office was also investigating reports that Islamic State militants had killed 40 civilians in one village.

Since the launch of operations on Monday to recapture the northern Iraqi city of Mosul from Islamic State, the U.S.-led coalition has stepped air strikes in support of the advancing Iraqi army and peshmerga forces.

The Turkish Red Crescent is sending trucks of aid to northern Iraq with enough food and humanitarian supplies for 10,000 people displaced by fighting in Mosul, the president of the agency said.

Iraqi and Kurdish forces have seized territory around Mosul in recent days in preparation for a long-anticipated offensive on the last major stronghold held by Islamic State in Iraq. Turkey has warned of a potential wave of refugees.

The United States is the main contributor to the air campaign against Islamic State, with over 80 percent of the air strikes.