More than 2000 marines were brought to Afghanistan in late April to hunt leaders of the Taliban and al-Qaida and destroy their networks.
"As of now I would characterise that we have inflicted in excess of 80 casualties on the anti-coalition militias since we've begun talking about this operation," US-led occupation force spokesman Lieutenant Colonel Tucker Mansager said on Saturday.
The suspected anti-government fighters have been killed in fighting over the past three weeks in offensives focusing particularly on Zabul province's Deh Chopan district, some 300km southwest of the capital Kabul.
Kandahar's military commander Khan Muhammad said most of the dead had been killed by bombings in mountains.
The marines are based in southcentral Uruzgan but operate frequently in neighbouring Zabul to the east and southern Kandahar province.
Last week US-led coalition troops carried out sweeps of the former Taliban strongholds as part of an offensive aimed at stemming a spiral of violence in the lead-up to elections scheduled for September.
On Thursday, 11 Chinese road construction workers were killed by gunmen in an overnight attack on their camp in northeastern Kunduz province. On 2 June, three Europeans and two Afghans working for the Medecins Sans Frontieres aid group were shot dead in northwestern Badghis province.
The marine's operations have also come against a background of renewed activity by the Pakistani army in the tribal regions along the countries' common border.
Mansager said there had been "no particular increase" in cross border activity in the southeast of Afghanistan since the latest Pakistani operation began.
Meanwhile, US marines also captured a bomb-maker believed to be a "medium value target" on Friday in an area some 70 kilometres south of Kabul, Mansager said, without giving any further clarification of the location.