"Right now, it's 40 years next year since the Soviet invasion", which lit the fuse for the bloody ethnic, sectarian and political rivalries that have continued through the Taliban's rise to power, he said. The great majority of USA forces withdrew by the end of 2014, but those still in the country often are involved in supporting combat situations on the ground as well as by air.
The Pentagon said the incident remains under investigation.
The deaths came just days after Sgt. Leandro Jasso, a member of the elite 75th Ranger Regiment, was killed battling al-Qaida fighters during a raid in southwestern Nimruz province.
About 15,000 USA troops are deployed to Afghanistan, with most involved in Resolute Support, a US -led North Atlantic Treaty Organisation mission to train, advise and assist Afghan forces battling the Taliban.More news: Melania Trump uses Christmas decorations to promote 'Be Best' campaign
"Three US service members were killed and three wounded when an improvised explosive device detonated November 27 near Ghazni city".
Three other US service members were wounded along with a USA military contractor, they added.
However diplomats said much of the focus will be on side meetings, where officials from Afghanistan and regional and Western countries will have a chance to assess the efforts of USA peace envoy Zalmay Khalilzad.
Mattis declined to characterize the progress made by Khalilzad, but said, "There's an active effort underway, and it includes folks from the Taliban side".More news: US Border Security Closes San Ysidro Port of Entry
The Taliban has claimed the attack that killed three US service members and completely destroyed a USA tank in eastern Afghanistan, AP reported. Ghazni was the only province that didn't hold parliamentary elections last month.
"You noticed President [Ashraf] Ghani announcing at least a total 29,000 Afghan troops killed since 2015". More typically, a general carries a pistol.
The U.S. and North Atlantic Treaty Organisation formally concluded their combat mission in 2014, but still provide vital support to Afghan security forces, who have been struggling in recent years to combat a resurgent Taliban and ISIS affiliate.
A total of 13 USA troops have been killed this year in Afghanistan, according to the Defence Post. "We have no conscription, nobody is forced, and if there was not a patriotic impulse, I don't think that people would sacrifice their lives for a pay of $200 [per month]".More news: Became known why Prince Harry will live with the mother