Mosul looks like a city of the dead. American and Iraqi troops have launched an attack aimed at crushing the last bastion of al- Qa'ida in Iraq and in doing so have turned the country's northern capital into a ghost town.

Soldiers shoot at any civilian vehicle on the streets in defiance of a strict curfew. Two men, a woman and child in one car which failed to stop were shot dead yesterday by US troops, who issued a statement saying the men were armed and one made "threatening movements".

Mosul, on the Tigris river, is inhabited by 1.4 million people, but has been sealed off from the outside world by hundreds of police and army checkpoints since the Iraqi government offensive against al-Qa'ida began at 4am on Saturday. The operation is a critical part of an attempt to reassert military control over Iraq which has led to heavy fighting in Baghdad and Basra.

The besieged city is now difficult to reach; we began the journey from the Kurdish capital Arbil in a convoy of white pick-up trucks, each with a heavy machine gun in the back manned by alert-looking soldiers, some wearing black face masks, that were escorting Khasro Goran, the deputy governor of Mosul, to his office in the city.

Soon after crossing the long bridge over the Zaab river and leaving territory officially controlled by the Kurds, we saw lines of trucks and cars being stopped by police. Their drivers presumably had not heard of the curfew. At the Christian village of Bartilla we exchanged our pick-ups for more heavily armoured vehicles with windows a few inches across of bulletproof glass.

I had been to Mosul down this road half a dozen times since the fall of Saddam Hussein in 2003 and on each occasion the military escort necessary to reach the city safely has grown bigger. Squinting through the small glass portholes it was clear that local people were taking the curfew seriously. Even the miserable cafes used by the truck drivers, and which I had imagined never closed their doors, had pulled down their metal shutters.

In eastern Mosul the streets are usually bustling and stalls spill on to the road near the tomb of the prophet Jonah, who died here some time after his alarming experience with the whale. Most of the people living in this part of the city are Kurds, who support the central government against al-Qa'ida. Yet, here too every shop was shut and there were police and soldiers at checkpoints every 50 yards. An extra brigade had been sent from Baghdad for the offensive along with special security troops to reinforce the 2nd and 3rd divisions.

Outside the police headquarters, the black vehicles of the Interior Ministry, each with a heavy machine gun and a yellow head of a tiger as an insignia on the doors, were drawn up in rows. American helicopters flew high overhead as well as drones for reconnaissance. There was the occasional burst of firing and bomb blast in the distance. The governor of Mosul, Dunaid Kashmoula, says the city "has come to be dominated by the leaders of al-Qa'ida as a result of the delay in the military operation" originally scheduled for earlier this year.

Nevertheless, the insurgents in Mosul have never held whole quarters of the city and there was no street fighting.

The Iraqi prime minister Nouri al-Maliki promised this offensive on Mosul as the last battle against al-Qa'ida. He promised revenge for the assassination of the previous police commander for the city who had been assassinated by an al-Qa'ida suicide bomber dressed in a police uniform.

These are critical days for Mr Maliki's government. Since 25 March he has launched military offensives in Basra and Baghdad. He is receiving support from the Americans and the Kurds. But it is not clear if the Iraqi army will fight without the backing of US firepower in the air or on the ground. On Saturday a ceasefire was agreed with the Mehdi Army of Muqtada al-Sadr in Sadr City giving the government greater control. But, as in Mosul, it is not clear how far the government's opponents have simply retreated to fight another day.

There is no doubt that security in Mosul has been deteriorating over the last six months. Mr Goran, who in effect runs the city, said that 90 people were killed in Mosul last September compared to 213 dead this March, including 58 soldiers and policemen. The number of roadside bombs had risen from 175 to 269 over the same period.

The official theory for this is that al-Qa'ida in Iraq, which has only a limited connection with Osama bin Laden and is largely home grown, has been driven out of its bastions in Anbar and Diyala provinces and Sunni districts of Baghdad. It has retreated to Mosul, the largest Sunni Arab city and the third largest in Iraq.

This is probably over-simple. Attacks on US troops in Anbar province have restarted and in Sunni districts of west Baghdad al-Qa'ida appears to be lying low rather than being eliminated. In many cases in Baghdad al-Sahwa, the supposedly anti-al-Qa'ida awakening councils paid by the Americans, in practice have cosy arrangements with al-Qa'ida.

I had decided to go to Mosul - arriving in the first hours of the government offensive - because of what proved to be a false report that the head of al-Qa'ida in Iraq, Abu Ayyub al-Masri, had been captured in the city. Later Iraqi security officers said they captured many "Emirs", al-Qa'ida cell leaders, and targeted hundreds of suspected houses.

I was in Mosul on the day it was surrendered by Saddam Hussein's forces in 2003. Scenes of joy were succeeded within the space of a few hours by looting and gun battles between Arabs and Kurds. Five years later Mosul, one of the great cities of the world, looks ruinous and under siege. Every alley way is blocked by barricades and the only new building is in the form of concrete blast walls. The fact that the government has to empty the streets of Mosul of its people to establish peace for a few days shows how far the city is from genuine peace.

© 2008 The Independent