General Richard Myers, chairman of the Joint Chiefs of Staff, said on Tuesday: "Where they are right now is where they were almost a year ago, and it's nowhere near the peak."
About half the attacks, which include bombings, shootings, rocket and mortar attacks, caused significant damage, injuries and deaths, Myers said.
But Myers and Defence Secretary Donald Rumsfeld disputed a suggestion that the figure showed a lack of progress in Iraq.
Rumsfeld said the focus of US forces had been training Iraqi security forces rather than directing counterattack operations.
Attacks exceeded 800 a week
just before the January elections
"The United States and the coalition forces, in my personal view, will not be the thing that will defeat the insurgency," he said.
"The people that are going to defeat that insurgency are going to be the Iraqis. And the Iraqis will do it not through military means solely, but by progress on the political side and giving the Iraqi people a sense that they have a stake in that country."
A US defence official, speaking on condition of anonymity, said post-invasion attacks in Iraq were at lower levels, 150 to 200 a week, until April 2004, when uprisings occurred in Najaf and al-Anbar province.
The first pictures of tortured prisoners from the Abu Ghraib prison were also made public that month.
Afterwards, the rate of attacks doubled to about 400 a week, the official said.
"Where they are right now is where they were almost a year ago, and it's nowhere near the peak"
General Richard Myers, chairman of the Joint Chiefs of Staff
The number spiked during US offensives in Najaf in August and in Falluja in November.
In January, as Iraq prepared to hold its elections, the number increased to as high as 825 attacks in one week, the official said.
Afterwards, the number dropped again to 400 a week in February and 350 in March, leading to hopes that the fight was finally ebbing because of the elections.
About 60 people, including Iraqis, Americans and other foreigners, are injured or killed daily in attacks, the defence official said.
Except for the spikes, that rate has remained roughly consistent since April 2004, the official added.