US-backed local forces are on the verge of recapturing major Islamic State (Isis) strongholds in both Syria and Libya, and are preparing to attack the Islamist group in its de facto capital of Mosul in Iraq.

A top US military commander has said that IS (Daesh) is on the retreat "on all fronts" and the number of its combatants at its disposal has declined. Lieutenant General Sean MacFarland said the military campaigns in Iraq and Syria have reduced the number of Isis fighters to as few as 15,000. MacFarland said fewer foreign Islamists are travelling to Iraq and Syria, and many people pressed into fighting for the Islamists are unwilling or untrained.

Isis on the back foot

Syrian democratic forces are on the brink of defeating IS in Manbij in the northern province of Aleppo. MacFarland said the city was largely under the control of US-backed forces, and the pockets of enemy resistance are shrinking daily. "I don't give it very long before that operation is concluded, and that will deal a decisive blow to the enemy," he said. Asked how long it will take, he said possibly a week or two.

The Syria Democratic Forces (SDF), which includes the powerful Kurdish YPG militia and Arab fighters, launched its campaign two months ago with the backing of US special forces to drive Islamic State from a last stretch of the Syrian-Turkish frontier. The official spokesman of the SDF-allied Manbij military council, Sharfan Darwish, told Reuters that battles were continuing but that around 90% of the city had now been cleared of the ultra hardline Sunni militants.

US-backed forces in Libya have liberated "70%" of the city of Sirte, the Islamic State group's last bastion in the north African country. Mokhtar Khalifa, the Sirte mayor, told The Associated Press that the city's southern and western sections are under control of the Libyan fighters loyal to the UN-brokered government in Tripoli, the country's capital. He estimated the number of Isis militants remaining in Sirte is in the hundreds.

Islamic State seized Sirte, the hometown of Libya's former dictator Muammar Gaddafi, in 2015. Libyan pro-government forces launched an operation to retake it in June 2016. After initial successes, the advance stalled because of snipers, roadside bombs and suicide bombers. At the beginning of August, US war planes joined the fight, carrying out a series of air strikes targeting IS positions in Sirte, breaking the stalemate.

General MacFarland said US-backed Iraqi forces are nearly ready to begin the fight for the northern city of Mosul. But he added that the US still has quite a bit of work to do at the Qayyarah Air Base in northern Iraq before it can be used as a hub for the battle to retake Mosul. US President Barack Obama authorised the deployment of 560 more American troops to Iraq to help transform the air base into a staging area for the eventual battle to oust IS from Mosul.

Once the fighting intensifies, up to a million people could be driven from their homes in northern Iraq, "posing a massive humanitarian problem for the country", the International Committee of the Red Cross said. More than 3.4 million people have already been forced by conflict to leave their homes across Iraq. Some residents of Mosul warn that getting rid of Dash won't solve the city's problems, as the Sunni-majority population would refuse to return to what they called the repressive yoke imposed by the Shi'ite-led government in Baghdad in the past.

MacFarland cautioned that while there have been successes on all fronts, IS will continue to be a threat. "Military success in Iraq and Syria will not necessarily mean the end of Daesh," he said. "We can expect the enemy to adapt, to morph into a true insurgent force and terrorist organisation capable of horrific attacks like the one on 3 July in Baghdad and those others we've seen around the world."