The US military confirmed for the first time today that four helicopters that crashed in Iraq in the past two weeks had all been shot down.
US military spokesman Major-General William Caldwell said US forces were adjusting their tactics and changing the way in which attack helicopters flew in support of Iraqi and American troops.
"Based on what we've seen, we are already adjusting our tactics and procedures in how we deploy our helicopters," he told reporters in Baghdad.
Meanwhile, the Iraqi government said today that half the Sunni Muslim militants behind the bombings shaking the country had arrived through neighbouring Syria.
The United States and Iraqi officials have long accused Iraq's neighbours, particularly Syria and Iran, of failing to stop Islamic militants from crossing into Iraq to carry out attacks.
US and Iraqi officials say many of the insurgents responsible for the violence are not Iraqis.
"We have confirmation that 50 per cent of these takfiris and killers who call themselves Arab jihadists come across the Syrian border," Iraqi government spokesman Ali al-Dabbagh told Al Arabiya TV, referring to radical Sunnis whose ideology brands some Muslims as infidels and allows their killing.
"Syria closes its eyes. As we have said before and we say again today, we are facing a bloody and painful day for us in Iraq as a result of Syria's lack of seriousness in controlling the border."
Iraq's Shia-led government today renewed its pledge to crack down on Saddam Hussein supporters and Sunni militants after a truck bomb killed 135 people in a mainly Shia area of Baghdad yesteday in the deadliest single attack since the 2003 invasion.
Around 1,000 people have been killed across Iraq in the past week in suicide bombings, shootings and fighting between militants and security forces.