WASHINGTON, December 15, 2004 (Reuters) - A bold, innovative insurgency has become more effective against U.S. supply lines in Iraq and explosive attacks have slowed military operations there, a senior American general has said.

"They have had a growing understanding that where they can affect us is in the logistics flow," Air Force Lt. Gen. Lance Smith, deputy chief of the U.S. Central Command, told reporters.

"They have gotten more effective in using IEDs (improvised explosive devices)," Smith added at Pentagon briefing. The insurgency "has become more effective."

Smith said U.S. forces in Iraq now totalled 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 that explosives placed beside roads were hindering military operations and reconstruction nearly two years after the U.S.-led invasion of that country.

"Yes, they are," Smith replied when asked if response to such attacks were slowing down operations. "They cause us to re-route vehicles. They cause us to have to employ tactics - although the tactics are generally successful - in avoiding them. And (they) cause us to have to convoy where maybe otherwise we would prefer to move in smaller numbers."

"So it is having an impact."