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Thousands of messages of love were dropped on western Mosul in an effort to show solidarity with around 750,000 civilians who are trapped in the Islamic State group-held territory.

The letters were among four million dropped over IS-held parts of Mosul since December by a non-profit organisation, The Institute for War and Peace Reporting.

"I just want to let you know what we are safe here. I hope your areas will be liberated by government forces as soon as possible and be free from ISIS fighters," read one letter penned by a writer by the name of Omar, who currently lives in a safe part of the city. "We are all Iraq, I love you."

Another read, "You will return to your most beautiful days with your family and you will hear the prayers of the mosques in Mosul sky, the culture and science will return because everything been robbed from you will come back to you."

The letters were written by Iraqis from across the country and then collected by volunteers.

Using transport planes, photocopies of the letters were then showered down on areas still held by the extremist group.

The United Nations estimated that about 750,000 civilians are trapped in western Mosul.

The initial numbers of displaced from western Mosul have been low, but Iraqi forces are yet to punch into the city's dense urban neighbourhoods.

The battle for western Mosul, the extremist group's last major urban bastion in Iraq, is expected to be the most daunting yet, according to Iraqi and coalition officials.

The western half of the city is denser with older neighborhoods and narrower streets that will likely complicate the already difficult urban combat ahead.