An international rights group called Sunday for an investigation into an Iraqi airstrike on a school housing displaced families that killed 31 civilians, including 24 children, a day after the country's new prime minister ordered the army to stop shelling militant-held populated areas to minimize civilian casualties.

New York-based Human Rights Watch said in a statement that the Sept. 1 airstrike took place in the town of al-Alam, just outside the northern city of Tikrit, which was captured by the Islamic State extremist group in June. The statement said 41 other people were wounded.

Three survivors, interviewed by HRW by phone, said the school sheltered about 70 people from the extended Jurefat family, who had been there for two months after fleeing Tikrit. They said that there were no militants or military equipment in or near the school at the time of the attack.

Iraqi officials were not immediately available for comment, but the report cited the Iraqi government as saying that the pilot had targeted a car driving near the school that was thought to be transporting fighters and explosives, causing an explosion that was "far larger than normal."

HRW special adviser Fred Abrahams said the Islamic State group "is incredibly brutal, but that's no excuse for what the Iraqi government is doing." He called on Iraq's allies in the fight against the militants "to put pressure on Baghdad to stop this kind of violence."

On Saturday, Iraq's newly-appointed Prime Minister Haider al-Abadi ordered the security forces to stop shelling populated areas held by militants in order to spare the lives of "innocent victims" as the armed forces struggle to retake cities and towns seized by the Islamic State group since early this year.

He accused the militants of using civilians as human shields to stop the advance of Iraqi security forces. But he vowed to continue military operations against the Al Qaeda breakaway group, which seized much of northern and western Iraq in an unprecedented June offensive.

Hours after the HRW statement was issued, four missiles hit the hospital of the militant-held city of Fallujah, 40 miles west of Baghdad, which has been encircled by government troops. One employee was wounded, a doctor said on condition of anonymity as he was not authorized to brief the media. The source of the shelling was unclear.

The army's heavy-handed tactics have long fueled anger among the country's Sunni minority, leading many to welcome the insurgents as liberators when they swept into Sunni-majority areas earlier this year. The Shiite-led government is under mounting pressure from the international community to reach out to both Sunnis and Kurds in order to form a united front against the militant onslaught.

Since Aug. 8, the U.S. has carried out more than 100 strikes using aircraft and unmanned drones. The American military has focused mainly on areas bordering the largely autonomous northern Kurdish region, where Kurdish forces have been fighting the militants, as well as strategic areas like dams.

The latest two airstrikes on Friday targeted Islamic State militants near the Mosul Dam, the U.S. military said. U.S. Central Command said the strikes destroyed a mortar emplacement and an armed vehicle, and brought the total number of strikes to 160 across Iraq since the military campaign began.

The onslaught by the Islamic State group stunned Iraq's U.S.-trained and equipped security forces, which melted away as the extremists advanced and captured key cities and towns in June. The militants also targeted Iraqi religious minorities, including Christians and followers of the ancient Yazidi faith, forcing hundreds of thousands from their homes.

On Sunday U.N. humanitarian chief Valerie Amos warned that more than half of the 1.8 million Iraqis who have been displaced are living in open areas such as parks, unfinished buildings and schools.

Amos added that children have not been able to return to school in the Kurdish Dahuk province because displaced people are still sheltering in more than 650 schools there.

"Winter is fast approaching and there is a huge amount of work needed to ensure families have protection from the cold," she said during a press conference held in Baghdad.

Al-Abadi called Sunday for a global fund to enable swift construction in areas that have been devastated by violence brought on by the Islamic State group.

Also Sunday, men armed with weapons fitted with silencers broke into the house of a member of an anti-militant group in the town of Madain, just south of Baghdad, killing him, as well as two men and two women, a police officer and a medical official said on condition of anonymity as they were not authorized to release information.

His group, known as the Awakening Council, or Sahwa, is made up of Sunni militiamen who joined U.S. troops in the fight against Al Qaeda during the height of Iraq's insurgency in 2007 and 2008.