Troops in Iraq vowed to step up their campaign to recapture Mosul from Islamic State (Isis) militants on Sunday, buoyed by their victory in Nimrud, earlier in the day. However, four weeks into operation, only the eastern defences surrounding the city had only been breached.

Recapturing Mosul will be vital in driving IS (Daesh) from Iraq, as the country's second largest city and the only remaining IS stronghold.

The campaign is the largest military operation in Iraq in the last decade, with 100,000 troops assembled by the Iraqi government, assisted by US air strikes.

Though progress was slower than anticipated, Iraqi army special forces officer Brigadier Ali Abdulla told Reuters the use of civilians as human shields by the militant group had held Iraqi troops back.

With Hadba, a town on the fringes of Mosul, now in sight, he did not expect the mission to gather pace immediately.

He said: "Our approach [to Hadba] will be very slow and cautious so that we can reach the families and free them from Daesh's grip," Abdulla said.

With IS having now lost most territory within Iraq, troops are seeing a sustained effort by the militants to keep a firm grip on Mosul, Abdulla said.

A range of methods such as suicide car bombs, roadside bombs, snipers and long range mortars were all in use to hold back advancing troops.

The use of a network of tunnels to launch surprise ambushes on the troops was also proving difficult to overcome. In some areas, control had changed hands multiple times as Iraqi troops struggled to lengthen short-term gains in the face of IS's combat tactics.

However, the earlier capture of Nimrud, just 30km from Mosul, had given Iraqi forces hope. Iraq's deputy culture minister Qais Hussain Rasheed said of recapturing the UNESCO-listed site: "Liberation of ancient Iraqi archaeological sites from the control of forces of dark and evil is a victory not only to Iraqis but for all humanity."

Mosul offensive
Smoke rises during clashes in the town of Bashiqa, east of Mosul, during an operation to attack Islamic State militants in Mosul, Iraq Thaier Al-Sudani/ Reuters