A wild dog was the first sign of life as Iraqi soldiers, supported by US troops, ventured into a village northeast of Baghdad.

Rifles raised, the group approached a courtyard of two mud-walled houses and a couple of huts, fearing that they could be rigged with explosives.

Instead, the buildings stand empty, all inhabitants and their belongings gone. Further searches throughout the rest of Fatamia reveal that only three or four families remained. Six months ago there were 30 to 40 families.

This eerie scene has been played out repeatedly in other villages across the southeastern corner of Diyala province, one of the country's most notorious areas. US and Iraqi commanders blame threats and intimidation by al-Qaeda fighters for the mass migration, but there is another reason for people to move into urban areas: severe drought.

Anxious to tame the province, the Iraqi Government announced two weeks ago that it was planning to launch a large offensive in Diyala. But the warning gave any insurgents hiding out in the province plenty of time to escape.

As a result, no big arrests have been made since the preliminary phase of the mission started last Friday — a frustration for the hundreds of US and Iraqi troops on the ground who must endure scorching temperatures as they trek from village to village.

“They should not have declared the operation on television because now all the top targets have got away,” one Iraqi officer said. “I think there are some people in the Government who are co-operating with the insurgents.” Mowaffak al-Rubaie, Iraq's National Security Adviser, said that announcing the offensive was a Government tactic to show the public who was in control, and to offer insurgents the chance to switch sides.

“On balance, detailed plans are kept secret while broad statements that have a positive psychological effect for the Government are broadcast,” he told The Times, adding that anyone who fled would probably be picked up at a later date. Advance notice was given before other operations, including in the northern city of Mosul and the southern province of Maysan. Both offensives took place without much of a fight. In contrast, violent clashes erupted when Iraqi forces launched a surprise attack on the main southern city of Basra last March.

Pushing south through Diyala, US and Iraqi troops set off at dawn with tanks, attack helicopters and hundreds of mounted troops. At first, progress was slow because the roads linking the villages and hamlets that dot this terrain are littered with roadside bombs, forcing the convoy to use armoured bulldozers to cut their own path through the rutted fields.

With no insurgents in sight, the next biggest threat became heatstroke. US soldiers carrying 27 kilograms (60lb) of equipment on their back drank at least ten litres of water each day, as well as energy drinks, to avoid dehydrating.

The heat was too much for one Iraqi soldier, who had to be taken by helicopter back to his base after collapsing in temperatures that passed 49C (120F). Several US troops were also treated in the field after showing signs of heat exhaustion.

After a day, the sweep through Fatamia village was largely complete. Captain Robert Green, commander of Grim Troop from “Sabre Squadron”, 2nd Squadron, 3rd Armoured Cavalry Regiment, allowed his men and about two dozen Iraqi soldiers to take a break in the backyard of a deserted school. He, like everyone, appeared slightly disappointed at the absence of insurgents.

“It would be nice to conduct an operation and snag a few bad guys. But if they are being made to move then we are still meeting our objective by removing their safe havens,” said the 36-year-old from Texas, on his third Iraq tour. Diyala, which stretches from the eastern outskirts of Baghdad to the Iranian border, is home to a variety of sects, including Sunni and Shia Arabs, Kurds and Turkmen. Such a mix was responsible for much of the tension and bloodshed in the insurgency that gripped Iraq after the invasion in 2003.

Al-Qaeda has continued to carry out attacks, notably suicide bombings involving women, in Diyala after being driven from other enclaves over the past year. In addition, breakaway factions of al-Mahdi Army, the main Shia militia, still hold sway in parts of the province.

The Diyala offensive — which Iraqi military officers have said will start in earnest in the coming days and involve some 30,000 police and soldiers — is part of a wider plan to underpin the authority of Nouri al-Maliki, the Prime Minister, before provincial elections that are due to take place by the end of the year.

Success on the security and political front is a key feature in talks between Baghdad and Washington on the long-term presence of the US military in Iraq. Mr al-Maliki is keen to see the back of US combat forces by 2010, but must demonstrate that his troops have the ability to take control on their own.

Mohamed Maroof Hussein, the Mayor of the nearest big town of Baladruz, said that many locals have fled their homes for fear of being unfairly arrested after rumours spread that Shia residents had been giving names of Sunnis to the authorities to get them into trouble — and vice versa. The Iraqis and Americans may have come prepared for a fight, but in the end it seems that the heat may have done their work for them. Tending to a small herd of sheep and goats, one of the few remaining villagers said that most people had left more than two months ago because of the drought, rather than problems with al-Qaeda.

“All of my crops failed because of the lack of rain. It is also hard to keep livestock,” said Hassan Selman, a married father of six, who is also planning to leave.

“The problem of security is only third on my list.”

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