A US helicopter has come down in southern Iraq after apparently coming under heavy fire from insurgents, according to reports by witnesses.

They said the helicopter came under attack near Latifiya, 40km (25 miles) south of the capital, Baghdad.

The US military confirmed a helicopter had gone down with four people injured. But it did not specify the cause.

The US has lost more than 50 military helicopters in Iraq since the invasion with the loss of a number of soldiers.

A major security operation in Baghdad has resulted in a decrease in the number of attacks on coalition troops there, the US military says.

However, the deaths of eight coalition soldiers were announced on Thursday - four British and four American.

President Mahmoud Ahmadinejad of Iran said Wednesday that he would immediately release 15 British sailors and marines who have been held captive in Iran since March 23.

Ahmadinejad said at a news conference in Tehran that he was giving the British military personnel amnesty and a pardon.

"I announce their freedom and their return to their people," he said. "They will be free after our meeting. They will go to the airport and will join their families."

The Iranian president said the decision to release the prisoners was not part of a swap with Iranian prisoners in Iraq.

"Our government has pardoned them, it is a gift from our people" he said. "It has nothing to do with this analysis. If we were to move forward on that basis things would have looked different. We approached the subject on a humanitarian basis. It was a unilateral decision on our end."

The announcement came after Ahmadinejad had taken the opportunity to pin a medal on the uniform of an Iranian naval commander who had captured the British sailors and marines.

The Britons were seized at gunpoint in disputed waters in the northern Persian Gulf. Iran contended that they illegally entered Iranian territorial waters.