Restrictions imposed on US-led forces in Afghanistan to reduce civilian casualties have also led to a decline in attacks by insurgent fighters, a new report says.

The recently fired US commander, General Stanley McChrystal, imposed the curbs last year on use of aerial bombing and other heavy weapons in a bid to lower the number of innocent people killed during operations against insurgents.

The rules also appear to have lessened the cycle of violence by reducing the number of insurgent attacks on US-led ISAF troops, according to the think tank the National Bureau of Economic Research, based in Cambridge, Massachusetts.

The 70-page report, which analyzes 15 months of data on military clashes and incidents totalling more than 4,000 civilian casualties, concludes that Afghan insurgents are to a large extent motivated by desire to avenge deaths of civilians.

"Civilian casualties are affecting future violence through increased recruitment into insurgent groups after a civilian casualty incident," the report says.

"Local exposure to violence from ISAF appears to be the primary driver of this effect."

"In Afghanistan," the report says, "when ISAF units kill civilians, this increases the number of willing combatants, leading to an increase in insurgent attacks."

The report comes amid political debate over the restrictions that McChrystal imposed and speculation that his successor, General David Petraeus, may relax the rules, leading to more violence against civilians.

US troops have reportedly complained about the restrictions, which bar aerial or mortar bombing of houses except in cases of immediate danger to soldiers.

Petraeus told the Senate during his confirmation hearing that he would "look very hard at this issue."

McChrystal put the curbs in place in a bid to win hearts and minds across Afghanistan, where Taliban guerrillas are holding down about 150,000 US and NATO troops.

Petraeus is credited with having brought a measure of stability in Iraq and is expected to bring some of the same counter-insurgency strategies to Afghanistan.

However the report from the National Bureau of Economic Research said there were crucial differences between the two countries in terms of how civilian casualties promote insurgent activity.

"In Iraq we find no evidence that civilian casualties affect long-run trends in violence," the report said.