KABUL — A top American commander today yesterday Iran of supplying powerful roadside bombs to militants in Afghanistan and said America would “act decisively” if the cross-border flow continues.

Heavy battles in the violence-plagued south, meanwhile, killed 75 Taliban and at least six civilians, and a suicide car bomb in the capital killed a French soldier and an Afghan bystander.

Admiral William Fallon, the head of U.S. Central Command, said Iran’s Revolutionary Guard is supplying roadside bomb parts for the type of sophisticated and deadly bombs found in Iraq known as explosively formed penetrators.

“The Iranians are clearly supplying some amount of lethal aid,” Admiral Fallon said during a trip to Afghanistan. “There is no doubt … that agents from Iran are involved in aiding the insurgency.”

Admiral Fallon said America was carefully watching the flow of weapons from Iran and said America would “act decisively” if the cross-border flow continues. His comments were not meant as a threat of military action against Iran but a suggestion that border interdiction efforts may need to be increased, Admiral Fallon’s aides said later.

Iran has denied that it is supplying arms to fighters in Afghanistan.

Admiral Fallon said Iran is also providing development assistance in western Afghanistan, which he labeled as helpful, and said its activities inside its eastern neighbor are meant to ensure that Iran has a role in the region’s politics.

“And I think they put a priority on causing us as much frustration as they can,” he said. “I think it’s all aimed at embarrassing us and one of their long-standing aims is getting us out of the region.”

NATO’s International Security Assistance Force has said that three shipments of weapons emanating from Iran have been intercepted in Afghanistan since April. The latest was discovered in the western province of Farah on September 6.

NATO’s top commander here, American General Dan McNeill, told The Washington Post in a story published yesterday that the September 6 shipment likely was sent into Afghanistan with the knowledge of Iran’s Republican Guard and possibly the Quds Force, the country’s elite covert military arm.

American military leaders have long said that Iran is supplying weapons to militants in Iraq that are used against American forces there.

Afghanistan has seen its heaviest fighting this year since the ouster of Taliban regime in 2001. More than 4,400 people have died in insurgency-related violence around the country, according to an Associated Press count based on official figures.

Yesterda’s bomb attack in western Kabul was directed against a convoy of French troops traveling in armored vehicles. It killed one soldier and an Afghan civilian and wounded many other Afghans, hospital and NATO officials said. The blast blew the windows out of a civilian bus and set at least one vehicle on fire.

President Sarkozy of France condemned the attack as “cowardly and odious.”

“More than ever, I remain determined to pursue the fight against terrorism,” he said.

Heavy fighting in the south, meanwhile, killed about 75 Taliban militants in the last 48 hours, the American-led coalition said.

On Wednesday, NATO launched a new operation in Helmand province, the world’s largest poppy-growing region, which has seen the heaviest fighting in Afghanistan this year.

Airstrikes were called in against “anti-coalition militants” in the Garmsir district early today, killing about 40 fighters, the coalition said. Soldiers found more than 20 rocket-propelled grenades, ammunition, and land mines in the militants’ compound, it said.

Six civilians, including women and children, died in a separate battle in Helmand province’s Gereshk region on Wednesday after Taliban militants fled fighting with NATO forces and sought shelter in the civilians’ homes, the Gereshk district chief, Abdul Manaf Khan, said.

NATO said there were “a number” of civilian casualties caused by the fighting.

Taliban fighters attacked coalition forces from a housing compound that was later targeted in an airstrike. NATO said it was “unaware” civilians were in the area.

Civilians deaths from America and NATO military action have become a major issue in Afghanistan this year. President Karzai has repeatedly pleaded with international forces to halt such casualties.

Afghan civilian deaths have declined since several incidents earlier in the summer. Wednesday’s deaths appear to be the first since early August.

In another newly reported battle, more than three dozen Taliban fighters were reported killed in a clash Wednesday in Uruzgan province, the coalition said.

The spike in violence came as Afghans marked the U.N.-backed International Day of Peace today. Marches and celebrations were held around the country, and a U.N. spokesman, Adrian Edwards, called the numbers of Afghans participating “remarkable.”

“I think there are two very different stories going on here. One is about peace, one is about conflict,” he said. “We’ve never seen anything like this (interest level) before.”