The crashes raise concerns that insurgents, who have proved highly innovative in warfare, have acquired new weaponry. Twenty Americans, including 16 soldiers and four civilians working for a security company, died in the crashes.

In the deadliest incident 12 soldiers died on January 20 when their Black Hawk came down near Baquba, northeast of Baghdad.

In each case, reports from witnesses suggested that the craft were shot down. Yesterday this was confirmed by Major General William Caldwell, the senior US military spokesman.

"We have had four helicopters shot down ... it appears they were all the result of some kind of ground fire," he said.

"Obviously, based on what we have seen, we are already making adjustments to our tactics and techniques, as to how we employ our helicopters."

Maj Caldwell said the threat went wider than the US military aircraft. Civilian security firms have also become increasingly dependent on the use of helicopters to move and protect employees. Five Americans from the Blackwater USA security firm died when their aircraft was brought down over Baghdad on January 23.

The confirmation that Iraqi insurgents have found a way of increasing their hit rate against helicopters poses US military strategists with a new headache just when they are under pressure to notch up successes under President Bush's so-called surge policy - increasing troop numbers by 21,000 in an attempt to regain the initiative in Baghdad and other key areas of the country.

Toby Dodge, an Iraq expert at Queen Mary's college, University of London, said that as the surge of US personnel gathered pace an increase in casualties from aircraft crashes was almost inevitable. US forces had already been pushed off the roads and into the air by roadside bombs, and as troops are moved around the battle zones in growing numbers they are becoming ever more vulnerable.

"This is the first sign that more American soldiers are being put in the line of fire, and the cost of that, unfortunately, is that these incidents are bound to increase," Mr Dodge said.

Evidence of the vulnerability of troops comes ahead of important Iraq debates on Capitol Hill this week. A resolution put forward by the Republican senator from Virginia, John Warner, would put the Senate on record as opposing the sending of combat reinforcements under the surge.

The US has had difficulty with attacks on its helicopters ever since Apache attack aircraft were deployed over Baghdad during the invasion in 2003, when they were found to be surprisingly vulnerable to ground fire. At the end of 2003 they began flying low and fast in order to avoid missiles launched from the ground, but this leaves them exposed to simple gunfire.

Insurgents use rocket-propelled grenades and anti-aircraft missiles in Iraq, making high flying impossible.

More than 50 helicopters have crashed since the invasion, about half under insurgent attack. Until the past two weeks it was felt that the flying tactics and use of varying routes had reduced the risk.

The most recent incident, last Friday, in which an Apache crashed north of Baghdad killing both its crew members, may be particularly troublesome to US strategists. Local reports suggested that two American aircraft came under fire and one was brought down after the pilot turned back to confront his assailants.

The helicopters had been travelling along a well-used route to the US base in Taji, suggesting that even channels that had previously been considered secure may now be vulnerable.

With the intensification of the American effort focused on Baghdad, tension remained high in the capital yesterday in the aftermath of the single most deadly bomb attack in the country since the start of the war. A suicide truck bomb on a Shia market on Saturday killed 132 people and wounded 302.

Sunni bombers are thought to be escalating their attacks in Shia areas ahead of an anticipated offensive by US and Iraqi forces on their strongholds in the capital. At least 22 other Iraqis died in a round of bombings and shootings in Baghdad yesterday.

Heavy toll:

January 20 Black Hawk downed near Baquba, killing 12 US soldiers.

January 23 Five US civilians die after security firm's helicopter crashes over Baghdad during battle

January 28 Apache with two soldiers downed during battle in Zaraq area

February 2 Apache brought down near Baghdad, killing two US soldiers