The three other American service members who were wounded in Tuesday's attack, one critically, are in stable condition after being evacuated to Forward Operating Base Salerno in nearby Khost province.
All three soldiers were members of 1st Battalion, 506th Infantry Regiment and had been in country since May.
The recent uptick in violence underscores the fierce fighting between U.S. and Afghan forces against Taliban and Pakistani-based insurgent groups in eastern Afghanistan.
Paktia, Khost and other volatile provinces in eastern Afghanistan that line the country's border with Pakistan were the last areas to be handed over from U.S. forces to Afghan National Security Forces (ANSF).
Afghan military and police commanders officially took the reins of the country's security operations from U.S. and coalition leaders in June.
These areas are also the primary focus for U.S. and allied commanders in Afghanistan for this year's fighting season, which will likely be the last for coalition forces in country.
The lull in violence in this year's fighting season that U.S. and Afghan leaders expected to accompany the Muslim holy month of Ramadan has yet to take effect in the eastern part of the country.
Attacks against U.S. and Afghan forces continue within the so-called Pashtun heartland of the Taliban insurgency, which runs from the eastern border provinces of Afghanistan into the southwest part of the country.
U.S. troops with the 1st-506th and their ANSF counterparts stationed at various combat outposts throughout Khost and Paktia have borne the brunt of those attacks.
Earlier this month, two other American soldiers with the same unit stationed at COP Chamkani were killed when a suicide bomber detonated explosives concealed within his motorcycle.
Prior to Tuesday's strike, American forces stationed at nearby COP Wilderness were hit with back to back rocket attacks, beginning on Saturday.
Local insurgents fired several 107mm rockets over the course of two days at the American outpost. No U.S. or Afghan army units stationed at the base were injured during the strikes.
The first 34,000 of the 66,000 U.S. troops in Afghanistan are slated to return home this spring.
The remaining 32,000 American forces will rotate stateside after the Afghan presidential elections in April 2014, ending the U.S. war in the country.
Washington and Kabul are negotiating terms for a possible residual force after 2014.
This story was last updated on July 18 at 5:59 a.m.