Air Force Lieutenant General Lance Smith, deputy chief of US Central Command, said on Wednesday that a bold, innovative "insurgency" is becoming more effective against US supply lines in Iraq and explosive attacks have slowed military operations there.
"They have had a growing understanding that where they can affect us is in the logistics flow. They have gotten more effective in using IEDs," said Smith, referring to improvised explosive devices hidden beside roads.
"They may use doorbells today to blow these things up. They may use remote controls from toys tomorrow. And as we adapt, they adapt," he added.
Smith said US forces in Iraq now totaled 148,000 troops - up from 138,000 at the start of this month and near the 150,000 planned to protect national elections in January.
But roadside explosives are hindering military operations and reconstruction nearly two years after the invasion.
Killing of troops
"They cause us to re-route vehicles. They cause us to have to employ tactics ... in avoiding them. And [they] cause us to have to convoy where maybe otherwise we would prefer to move in smaller numbers," Smith said. "So it is having an impact."
US Defence Secretary Donald Rumsfeld and the military have been sharply criticised by members of Congress and defence analysts for not anticipating the anti-US violence and its explosive attacks against convoys that have killed hundreds of US troops in the past year.
The commander of the US Air Force announced on Tuesday that the military in Iraq had begun using C-130 military cargo aircraft to ferry some food and equipment high above dangerous roadways in order to relieve pressure on ground convoys.
"They [anti-US fighters] have had a growing understanding that where they can affect us is in the logistics flow. They have gotten more effective in using IEDs [improvise explosive devices]"
Air Force Lieutenant General Lance Smith,
Deputy chief of US Central Command
On Capitol Hill, Senator Evan Bayh of Indiana, a Democrat on the Senate armed services and intelligence committees, said Congress should look into why an assessment of the situation in Iraq by the top CIA official was very different from opinions expressed by the US ambassador in Baghdad, John Negroponte.
"When you hear dramatically different opinions and assessments by officials of our government about the situation in Iraq in a matter of minutes, that bears further investigation as to why," Bayh said after a visit to Iraq.
The New York Times reported last week that the CIA's station chief in Baghdad had painted a bleak picture of Iraq in a classified cable and cautioned that security was likely to deteriorate unless the interim Iraqi government made significant progress in asserting its authority.
Bay said: "The ambassador obviously has to try and put a better face on things and with some reason, there are other things going on that do give some reason for hope."
At another Pentagon briefing, officials said the army was spending $4.1 billion in an accelerated effort to provide protective armour for Humvee jeeps, trucks and other military vehicles in Iraq.
They said all such vehicles, including cargo trucks, were expected to have factory-installed or other armour kits by next June.
Rumsfeld (L) has been criticised
over military strategy in Iraq
The "insurgency is a very, very sophisticated enemy", Army Major General Stephen Speakes said. "A year ago, the amount of explosive that was being used in an IED was much less than it is now."
His comments came after a US soldier complained to Rumsfeld in Kuwait last week that troops had to scrounge for scrap metal to protect their vehicles.