TIKRIT/ANKARA - Coalition air strikes are needed to retake Tikrit, where die-hard militants are defending their last redoubt with trenches, sandbags and roadside bombs, Iraqi officers said Sunday.

Two weeks into Baghdad’s biggest operation yet against the Islamic State (IS) group, Iraqi forces have a complete stranglehold on the city but are unable to launch a final assault, they said.

Staff Lieutenant General Abdulwahab al-Saadi said he had asked the defence ministry to request coalition involvement but “no air support” from foreign allies had yet been provided. Asked if US-led coalition air strikes were needed, Saadi said: “Of course... the Americans have advanced equipment, they have AWACS (surveillance) aircraft.

“They are able to locate the targets exactly” and carry out accurate strikes, he told AFP in an interview at Tikrit University on the northern edge of the city. “With the advanced technology of the aircraft and weapons they have, of course (strikes) by them are necessary,” Saadi said.

Saadi said that support from the Iraqi air force had been “limited” and not always sufficiently accurate. Fighters from the Imam Ali Brigade, a Shiite militia involved in the Tikrit operation, complained to AFP that a Sukhoi jet had even bombed pro-government forces by mistake. Since IS fighters took the city in June 2014, they have planted bombs underneath every road, according to residents who fled Tikrit.

One police officer gave an estimate of 10,000 IEDs (improvised explosive devices) in Tikrit, making any military advance perilous. “We are reinforcing our offensive capacities in the areas we have cleared and reinforcing our control on the entrances to the city,” an army major general said Sunday. “IS is putting up sandbags and digging trenches,” he said.

Karim al-Nuri, spokesman of the volunteer Popular Mobilisation units, said on Saturday that he expected Tikrit to be liberated within 72 hours. But the Iraqi army was mostly less upbeat, with one senior officer saying he could envision the battle of Tikrit lasting two more weeks. Saadi said he thought the reason there had been no coalition air strikes on Tikrit was political, not military.

Iran has been Baghdad’s main foreign partner in the operation and Tehran’s top commander in charge of external operations, Qassem Soleimani, has been omnipresent on the front lines. Officials in Washington have expressed unease at the level of Iranian involvement in Tikrit, an overwhelmingly Sunni city which was executed dictator Saddam Hussein’s hometown.

Coalition air strikes have supported several other operations to reclaim militant-held territory in Iraq in recent months, including some in which Iran-backed Shiite militias were involved.

Iraqi security forces, backed by Shiite volunteers and militias, and in some cases Sunni tribesmen opposed to IS, have in recent months been working their way north. In October, they retook Jurf al-Sakhr, the southernmost area to have been captured by IS, and have since also reclaimed the eastern province of Diyala. Iranian support was crucial in both those operations.  Kurdish peshmerga have also been pushing back militants from northern parts of the country, with US-led coalition support.

Meanwhile, three male British teenagers planning to join Islamic State militants in Syria have been detained in the Turkish city of Istanbul, Turkish and British officials said on Sunday.

The three, who have not been named, were detained on Friday, Turkish sources told Reuters, after a tip-off from British authorities that two of them were travelling to Turkeyvia Spain. Their detention comes after three British schoolgirls entered Turkey last month and are thought to have joined Islamic State in Syria.

Police in London said they had been made aware that two 17-year-old boys had gone missing and were thought to be travelling to Syria. Further inquiries revealed they had travelled with a 19-year-old male, police said. “Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria. They remain in detention in Turkey. The families have been kept informed of developments,” the police statement said.

Security services estimate some 600 Britons have gone to Syria or Iraq to join militant groups, including the man known as “Jihadi John” who has appeared in several Islamic State beheading videos. Hundreds of other Europeans have also joined the fight. Their involvement has raised fears about the possibility of attacks at home if they return trained and further radicalised.

Turkey meanwhile has faced criticism for not better controlling its southeastern borders, and has accused European countries of failing to prevent would-be militants from travelling in the first place.

Arrangements were being made with British authorities to deport the males this week, a senior Turkish official, who spoke on condition of anonymity, said.

On Thursday, the Turkish foreign minister said the girls who had earlier travelled to Syria had been helped to cross the border by a spy working for one of the countries in the U.S.-led coalition against the militants. Islamic State controls swathes of territory in Syria and Iraq where it has declared an Islamic caliphate.