Buoyed by recapturing Falluja from the Islamic State and the seizure of an airbase on Saturday, US and Iraqi officials are intensifying plans for an assault on Mosul, the terror group’s last urban stronghold in Iraq.



The US will send 560 more troops to the newly taken base, around 40 miles south of Mosul, which will be used as a staging point for the coming battle that officials suggest is likely to be launched later this year.



The fight for the country’s second-biggest city will define the fate of Isis in Iraq. The group emerged from civil war more than a decade ago and soared to prominence when it seized large parts of the country in mid-2014.



Since capitulating to Isis in Mosul, Iraqi forces have gradually regrouped and have taken back Tikrit, Ramadi and Falluja. All battles were strongly backed by US-led airstrikes.



The boost in troop numbers takes to 4,647 the number of US forces who have returned to Iraq to fight Isis. As Iraqi forces, aided by Shia irregulars, have gained traction on the battlefield, American advisers have started embedding with Iraqi brigades and battalions, drawing the advisers closer to the fighting.



In a visit to Baghdad on Monday, the US defence secretary, Ash Carter, said: “These additional US forces will bring unique capabilities to the campaign and provide critical enabler support to Iraqi forces at a key moment in the fight. Iraqi security forces, accompanied and advised by us as needed, will complete the southernmost envelopment of Mosul. That’s its strategic role, and that’s its strategic importance.”



The fight for Mosul will be the most complex in the campaign to claw back territory lost to Isis. The city is surrounded by villages from all approaches and the group has consolidated much of its leadership and many of its diehard fighters amid the city’s dense urban landscape.



Kurdish Peshmerga forces are also part of attack planning and have taken up positions to the north and south-east of the city. The Peshmerga and Iraqi troops occupy the same frontline near Makhmour, around 50 miles south-east of Mosul. As they have inched forward, both have been heavily supported from the air and by US artillery fired from a mountain behind them.



Iraqi troops say air support was decisive in the fight for Falluja, where up to 1,000 Isis fighters held out for six weeks against a sustained ground assault.



“The warplanes were perfect,” said Capt Ali Kazwini, an interior ministry official in Falluja. “If it wasn’t for them, we would still be here a long time.”



After being criticised for being too slow and sparing over the past two years, air support is proving decisive on the battlefields of Iraq, where the swath of the country held by Isis has dropped from more than 30% in late 2014 to an estimated 12% now.



In recent months in particular, Isis units have been battered by precision strikes that have destroyed weapons caches, bases and large numbers of fighters. Sunni leaders in Iraq, however, have cautioned that although Isis faces defeat militarily, it will remain a drawcard for Sunnis disenfranchised over the 13 years since the ousting of Saddam Hussein, and who have since faced a political process dominated by Iraq’s Shia majority.



