LIVE FOOTAGE: Mosul push ‘big moment’ in war on ISIS
The effort to oust Islamic State (IS) fighters from their Mosul stronghold in Iraq is a "big moment" in the war against the extremist group, Defence Secretary Sir Michael Fallon said.
Concerns have been raised by aid agencies that the long-expected attack has left civilians trapped in Iraq's second-largest city facing an "impossible decision" on whether to attempt to flee or seek shelter as the military operation happens around them.
British aircraft have been providing "intensive" support to Iraqi troops in the build-up to the Mosul operation and Sir Michael said the UK would continue to play a "leading role" in the fight against IS.
But he acknowledged the Iraqi forces faced a "tough" task operating in the "complex" urban environment.
More than one million people, including hundreds of thousands of children, are believed to be in Mosul, which has been under IS control for two years.
Safe escape routes out of the city for civilians caught up in the bloody conflict "do not exist", Save the Children has warned.
Families and children have been advised by local forces to stay inside and erect a white flag on their homes in a bid to stay safe, the charity's spokesman, Ruairidh Villar, said.
Speaking from Irbil in northern Iraq, he told the Press Association: "Whether families and children stay or they flee, right now it's an impossible decision."
Save the Children, which said humanitarian agencies are able to get no closer than 30 miles (48km) to the city, is calling on people to urge the Government to do what it can to ensure there are safe pathways for people to leave the area.
He said: "We are calling on the UK Government to put pressure on the Iraqi and coalition forces around Mosul now to establish safe corridors, safe escape routes for children to get out."
The UK Government has said the creation of such safe passages for civilians "hinges entirely on the good will of all parties to the conflict" and said this is unlikely where IS, also known as Daesh, is concerned.
In the days leading up to the launch of the offensive, RAF aircraft struck a series of IS targets around Mosul.
Tornado and Typhoon jets and Reaper drones have been involved in strikes, including against tunnel complexes, weapons caches and IS vehicles.
The militants are believed to have prepared for the expected onslaught by digging trench networks and setting booby traps.
On Sunday, the eve of the Iraqi offensive, a British Reaper drone targeted an artillery piece while Typhoons attacked IS positions to the east and west of the city.
Sir Michael said: "Daesh are on the back foot. The beginning of the encirclement of Mosul today is a big moment in our efforts to rid Iraq of Daesh.
"Mosul is a large and complex city and operations there will be tough but with coalition support Iraqi forces will prevail.
"Alongside our coalition partners, the UK will continue to play a leading role in the air and on the ground, including through our strike missions, specialised surveillance, humanitarian support and the mentoring and training of Iraqi forces."
The head of the UN's refugee agency Filippo Grande warned that protecting civilians must be an "essential part of military strategies" in the city.
He said he had raised the issue of protection civilians with the Iraqi government and received the "strongest assurances" from prime minister Haider al-Abadi.
But Cornwall-based charity Shelterbox said not knowing which locations people might flee to creates difficulties in organising aid provision, and makes it "a matter of instinct and assumption".
Rachel Harvey from the charity, who is in Irbil, about 85 miles (137km) from Mosul, said: "The aim is to get aid to displaced families as quickly as possible. So we are pre-positioning stock close to places where we think they might arrive. They are likely to be exhausted and traumatised by their recent experiences.
"Giving people shelter and essential items such as a solar lamp, blankets and a water carrier will allow them a degree of dignity and security to rest and recover."
Shadow international development secretary Kate Osamor said: "As the combined offensive of the Iraqi government and Kurdish forces against the Daesh occupation of Mosul makes important progress, everything possible must be done to safeguard the lives of the hundreds of thousands of civilians who live there.
"The needs of women, men and children fleeing Mosul, as well as those trapped inside the city, need to be addressed urgently in order to prevent a catastrophic humanitarian crisis in the area.
"It is important that all sides engaged in this conflict take every step possible to ensure that civilian lives are not put in peril and provide unfettered access to humanitarian aid."
International Development Secretary Priti Patel said the protection of civilians must be a "paramount concern".
She discussed the situation with UN emergency relief co-ordinator Stephen O'Brien.
Ms Patel said: "Retaking Mosul will be an important step towards defeating Daesh in Iraq and ending its tyranny over the civilian population in the city.
"However, with up to one-and-a-half million people still living in the city, it is clear that their protection and wellbeing must be a paramount concern.
"As I discussed with Mr O'Brien, we urge all parties in this conflict to ensure aid agencies get the access and support they need and that international humanitarian law is upheld.
"The government of Iraq, international community and aid agencies must work closely together to co-ordinate their plans.
"I made clear that we will also continue to call on other donors to follow Britain's significant lead and ensure sufficient funding to meet the expected scale of humanitarian need."