BAGHDAD -- Mosul's children are bearing the brunt of the intensified fight between U.S.-backed government forces and the Islamic State of Syria and Iraq (ISIS) group in the city's western half, the United Nations children's agency warned on Monday.

Iraqi forces are in their last push to drive ISIS militants from the remaining pockets of territory they still hold in the Old City where narrow streets and a dense civilian population are complicating the fight.

The UNICEF Representative in Iraq, Peter Hawkins, said the agency is receiving "alarming reports" of civilians being killed, including children, with some caught in the crossfire while trying to flee.

Hawkins didn't give a specific number for killed children.

Get Breaking News Delivered to Your Inbox

He estimated that 100,000 girls and boys are still in the ISIS-held Old City neighborhood and other areas, living under extremely dangerous conditions. He called on the warring parties to "protect the children and keep them out of harm's way at all times, in line with their obligations under humanitarian law."

"Children's lives are on the line. Children are being killed, injured and used as human shields. Children are experiencing and witnessing terrible violence that no human being should ever witness," he said in a statement. "In some cases, they have been forced to participate in the fighting and violence," he added.

Backed by the U.S.-led international coalition, Iraq last October launched a wide-scale military offensive to recapture Mosul and the surrounding areas, with various Iraqi military, police and paramilitary forces taking part in the operation. The city's eastern half was declared liberated in January, and the push for the city's western section, separated from the east by the Tigris River, began the following month.

Meanwhile, an international human rights group reported Monday that at least 26 bodies of "blindfolded and handcuffed" men were found in government-controlled areas and around Mosul since the operation started.

Human Rights Watch said local armed forces told foreign journalists that in 15 of the cases the men were extra-judicially killed by government forces who were holding them on suspicion of being affiliated with the ISIS.

HRW added that in the remaining cases, that were reported by local and international sources, the sites of the apparent executions all in government held territory raise concerns about government responsibility for the killings.

"The bodies of bound and blindfolded men are being found one after the other in and around Mosul and in the Tigris River, raising serious concerns about extrajudicial killings by government forces," said Lama Fakih, deputy Middle East director at Human Rights Watch. "The lack of any apparent government action to investigate these deaths undermines the government's statements on protecting detainee rights."

Extrajudicial executions during an armed conflict are war crimes and if widespread or systematic, carried out as part of policy, would constitute crimes against humanity, it said.

Iraq's second-largest city, Mosul fell to ISIS in the summer of 2014 as the militants swept over much of the country's north and central areas. Weeks later the head of the Sunni extremist group, Abu Bakr al-Baghdadi, announced the formation of a self-styled caliphate in Iraq and Syria from the pulpit of a Mosul mosque.