The leader of the group The Islamic State of Iraq has said that the country has become "the university of terrorism".
Omar al-Baghdadi said that fighters belonging to his group, which is associated with al-Qaeda, have used the war in Iraq, to achieve high levels of training, in an audio recording released on the internet on Tuesday.
"From the military point of view, one of the [enemy] devils was right in saying that if Afghanistan was a school of terror, then Iraq is a university of terrorism," al-Baghdadi said in the recording.
"The largest batch of soldiers for jihad in the path of God in the history of Iraq are graduating and they have the highest level of competence in the world."
Baghdadi, speaking to mark the fourth anniversary of the US invasion of Iraq, also said that his group was increasingly confident of success.
"The fear of the American marines has disappeared from the hearts of the people of the world, as the mujahidin have become thousands from the few they were after the fall of the infidel Baath regime," he said.
"These are just some of the achievements of four years of jihad."
The authenticity of the tape could not be verified but it was posted on websites previously used by al-Qaeda and other insurgent groups in Iraq.
Acid truck bombing
On Tuesday, the US military said that anti-government fighters in northern Iraq had tried to attack US and Iraqi troops with a lorry filled with explosives and nitric acid.
The attack failed when the lorry overturned before it reached its target north of Baghdad.
When US soldiers when to help the driver, they found the truck filled with eight containers of nitric acid and explosives, the military said in a statement.
The driver said he had been paid to attack the joint security station in Mushada, a town north of Baghdad, the statement said.
New US casualties reported
The US military reported the deaths of five troops in a statement issued on Tuesday morning.
Three soldiers were killed on Monday, two in Iraq's western Anbar province and another in Baghdad.
Two others were killed in Anbar province on Saturday when their vehicle was struck by a roadside bomb, the statement said.
More than 50 US troops have died in Iraq so far this month.