The number of airstrikes in 2005, running at a monthly average of 25 until August, surged to 120 in November and an expected 150 in December, according to official military figures.

The tempo looks set to increase this year as the Americans pull back from urban combat, leaving street fighting increasingly to Iraqi forces supported by US air power.

“The bottom line will be that as the Iraqi army and police gain in competence, they will be able to take on more and more of the territory,” said General Peter Pace, chairman of the joint chiefs of staff, announcing a cut in troop numbers from 160,000 to fewer than 138,000 by March.

The intensification in the air war comes as Iraqi politicians struggle in the aftermath of last month’s elections to put together a coalition government that will satisfy the disaffected Sunni minority, which ran Iraq under Saddam Hussein.

“We are insisting on a national unity government,” said Adel Abdel Mahdi, a leading member of the Shi’ite United Iraqi Alliance, the likely election winner.

The Sunni bloc, allied to the secular party of Iyad Allawi, the former prime minister, has been secretly discussing the terms of a possible political deal with insurgent groups. But those groups’ leaders have a long list of demands, chief of which is a timetable for American withdrawal as well as the release of prisoners, an effective rehabilitation of Saddam’s former ruling Ba’ath party and the disbanding of Shi’ite and Kurdish militias.

Insurgent sources said that they are also including Al-Qaeda in Iraq in their talks as its involvement was vital if a deal was to work.

President George W Bush promised in a pre-Chistmas speech that America will leave Iraq only when “victory” has been achieved, but the term is being quietly redefined.

Dov Zakheim, a senior Pentagon official during Bush’s first term in office, said: “The goal is not democracy, it is a united Iraq that doesn’t bother its neighbours. There is no law that says American troops have to be in the most hostile areas.”

The shift to air power is part of that policy. Determined to reduce “collateral damage”, the American military is relying on laser or satellite-guided bombs that can strike rooms or buildings without killing large numbers of civilians.

The bombs are also getting smaller: 500lb devices are becoming the norm, rather than those of 1,000lb or 2,000lb common in recent conflicts, and 3,000 new 7in 250lb devices are on order. Allen Peck, a US air force general, said that in some cases the 100lb Hellfire missile is used: “It won’t knock down a house, but it can be effective in taking out a car.”

In an example of the strategy, two US F16 fighters last week dropped two 500lb laser-guided bombs on three men planting roadside explosives in Kirkuk province, killing them and seven others.

However, some experts insist that even the smallest, most precise bombs cannot replace boots on the ground.

“It’s transitory. You hit it, even occupy it, but then the insurgents return when you’ve gone, like Falluja last year,” said Wing Commander Andrew Brookes of the International Insititute for Strategic Studies.

“Even a 400lb bomb has a wide area of blast and you are quite likely to kill some civilians. Kill a wife, children, mother or uncle and people become so angry the terrorist cycle starts all over again.”

There is also concern that Iraqi forces could abuse US air power. In a recent issue of The New Yorker magazine, a senior Pentagon military planner wondered aloud to Seymour Hersh, the writer: “Will the Iraqis call in airstrikes in order to snuff rivals, other warlords or members of their own sect and blame somebody else? Will some Iraqis be targeting on behalf of Al-Qaeda, the insurgency or the Iranians?”

The Americans insist, however, they will keep control of targeting by “embedding” more US troops in Iraqi police and army units. At the same time they are making no effort to build an Iraqi air force — a sign that they have no intention of ceding control of the skies to a new Iraqi government.

If the use of planes proves effective, US troop levels should fall below 100,000 by next autumn in time for the American mid-term congressional elections. The US death toll in Iraq last year was 841 — just five short of the 2004 total.

“The biggest problem we have is that our strategy has to include winning the war at home,” said Zakheim. “We have a different electoral timetable to the Iraqis.”

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